Can't explain of Gomo/Eir suddenly bad speed.

Sorry to say, but if trust comreg maps you unlikely can expect 5G. Also, in certain situations 4G might be much better that 5G
According Comreg, your nearest Eir 5G mast(your precise location dependent) is in Ballyduvane and coverage(outdors) quite patchy around Rosscarbery. 5G @8km distance - highly unlikely, unless you have external antenna that work on those frequencies.
Your town has one and surrounded by several Eir 4G capable masts - Beduf, Caherbeg. @4km distance you have good stance, but i'd still recommend external antenna.

Locate yourself on Siteviewer map and see what mast are closest to you and what services they carry. Take it from there.

services on masts:
GSM=2G
UMTS=3G
LTE=4G
NR=5G

Unlikely that the only mast in town would be decommissioned, but what do i know...
Wonderful, right in the mid of town 😊 . does it carry Eir? - no idea, check what coverage you have next time you in town.
comreg has precise locations - gov institution after all, but might be outdated too
cellmapper will/might lack precision due to data available from users running app and not the officials. Does not look like there are many cellmapper users in your area

What device you have ?

As far i am aware, eir is the only operator whos site ID nomenclature has resemblance in siteviewer and cellmapper, you can check other areas where more data available in cellmapper

As mentioned, cellmapper accuracy could be miles off if insufficient data provided, but with eir ID's should be easy to pin-point.
Also, comreg site(actually both, cellmapper display dates first/last seen) could be well dated and lack of new ID's or still display ID's that no longer in service

I am not familiar with antenna you linked, but judging by price and lack of technical info i would not touch it with stick. seem waste of 20 to me.
At distances under 1.5-2 Km you might get some improvement with omni-directional
Distances over that i would recommend directional antenna for best results.
i have two sets and can confirm that they actually work great:
Iskra P58 (4G only)
Iskra P60 (4G and 5G)
Other options available, but usually you get what you pay for
disclaimer: not advertising seller, but had good interaction including replacement shipped ondamaged goods without much fuss.

When i was using these antennas (no longer as new mast build just 1/2 mile away) i was pointing to mast ~4Km away, getting speed in excess of 250DL/50UL
!!This does not guaranty your experience will be same!!

B3(band 3) is best runner for me at the moment, but its "mid-range" runner, where B20, B28 would go bit further however with lower speeds(seems what you get now).

Good deal depend on router in use (brand/model you have?). CA (carrier aggregation) come to play where you can gain speeds due to ability to use several bands at same time, providing mast does have multiple bands available.
I am currently use Huawei B818 (4G only) and ZTE 801A(5G) and… - ready… 4G beats 5G by good bit 😉, but this apply to me only due to various factors outside my control.

hope this helps

cellmapper list "registered" bands, comreg dont, not all masts have same bands. Make few circles around your area with cellmapper app - 1km radius from your place, 2km, … once data uploaded, might give you more info about cell/band availability in the area.
Router does not have to be brand new - i have secondhand for last n years and no complaints. For you - dont think you need to chase 5G, not for near future as it looks.
Eir is not the only provider for data, you know - perhaps worth a test with other providers (beware of unlocked router/phone required for testing/use)
With huawei router you could install and run LTE H-monitor - this (free version, dont have to pay) tool allow to "lock" on desired bands(providing bands available in the area, might require re-locking once in the while). other apps available, only mention what i use/know.

From two decades or so ago, but still valid - placing router up high with minimum obstructions might help with signal/speed - have access to the attic?

Your signal stats seem OK providing distance to the mast (what is it, give or take 2Km ?)
42210 does not seem to appear on cellmapper - did you upload data from your app?

how much mobile data have you done in this billing period? by chance you hit the ceiling (120GB) and now throttled to 5Mbps?

10cm is a lot ! 😊
TBH i went through all this before i got antennas - move/turn/rotation of router on any axis could have impact of some degree.

Now that you have Hmonitor, you can move router around and see if there is better spot
Hmonitor display what mast(eNB ID), cell and band you actually connected with router - is it still 42210 or 43200 or different?

as far i can see , cartoradio covers french territories only, they don't seem to expand to other EU countries on continent or ROI

As per table above (i take it from your router) you are connected to 42210 on B20, comreg indicate this site located in Caherbeg and i dont see reason to think different
This is all that you know about 42210. there could be other bands on it, but unless you circle the place few times with cellmapper on your phone and upload data you wont know more.

on Hmonitor, force your router UL band into B3, see if it get connection and if it does see what speed you get.

Gomo support should be able to tell you if you over limits. bit pointless troubleshoot speed issues in case you are capped by ISP.

If you based north of Ardagh East, Rosscarbery does not look appealing even if you closer to it due to terrain.

I do not encourage you to disclose any information that might expose your personal information including your location. Should you choose to do so - PM, but even then i can't ensure it wont be read by other parties(board's admin/mods/etc/hell-know-who-else). Where i have no intentions to cause you any harm by sharing out info given to me or any other way, you should not trust random person on the internet public forum even if it would be hosted in ROI, let alone abroad. Be wise. You have one day(24h , mods help after this term) to remove data from you post and let me know (PM) you want me to cut/edit some from mine.

Now that we done with conspiracies… :D

You never stated brand/model of your router - are you still on tp-link mr500 ? If you are, i cant comment how software would behave even if its able to connect to it and read some info. hmonitor , according its producers, is designed to work with huawei brand only. B3 missing is not promising much, but we deal with what we have.

Despite you have disabled/not enabled data upload to cellmapper, you now know that Rosscarbery mast ID is 43524 and this indeed correspond to comreg EIR_CK_3524, you still don't know all the bands available there. Cellmapper reads not only cells that your phone is connected to, but neighboring cells(bands) as well. Its your free choice to upload data or not .

All below is nice presentation with images, but real life situations could prove all that wrong.
I am not radiowave wizard, just a fella that went through same hassle back a decade ago as you are right now. Dont mind share my knowledge/experience, but it will be you to test and prove me wrong :D

If we hypothetically take your location is rosalithr B&B(green pointer for all below), you have huge lump of rock on the way to Rosscarbery and this does not take consideration of buildings and trees.

Same time path to caherbeg is more/less clear

Where it does not eliminates your possible connection to rosscarbery, i think signal would be stronger from caherbeg.
Benduff is an option with clear path, but double distance and without directional antenna could be overwhelmed by signal from caherbeg.

Note: before commitment to buy, please read mounting instruction of antenna's that have linked(or any other) - there are specifics

As for your neighbor - congestion is a factor, but i dont think to that degree by one odd person/connection.

Did you try stick your router out the window?

Check and ensure router not set for external antenna

Record eNB and cell ID's, might become handy later.

Go closer to caherbeg or even all three sites and see if you able to register B3 or any other bands. Problem is that you dont know what tower and cell(band) you were connected while you had good/better speed

They all important, see here, search web for more info, perhaps here
signal metrics could be , how to put it …
I would accept compromise in some instances - say you could have average/poor signal metrics on B3 (as long you can maintain that connection ), but your speed could be higher than with better signal metrics on B20/B28

BTW, disregard signal "bars" on the phone - those are controlled by ISP and ISP decide to show 2-3-4 bars within their firmware. dBm are real values , just not simple to interpret.

Without antenna - dont see how this could work.

With antenna - trial-and-error, still dont like that hump on the way to Rosscarbery. Router decides what cell to connect based on signal strength. With some software(like hmonitor) you have ability to force specific band(cell) , but this providing that that signal is present - you say B3 not an option for you.
Also, not enough data on cellmapper to confirm that there is B3, caherbeg 42210 (EIR_CK_2210) is missing altogether.

try hmonitor again, reload if needed , see if settings you have are right, try different. Mine looks like this(note list of bands under upload). I am forcing UL to B3, DL is selected to B1,3,20,28 - most common, but not all sites might have all of them .

Observe events tab - logs would indicate if your router switching between cells/masts. record info(export) as its cleared once app closed.

Also, try to run x5 tabs of speed test at the same time or download big file (2-4GB). I noticed that my "casual" speed could sometime settle to DL40/UL60 and boost up 180/60 if i apply "load" (this where CA kick-in)

If your router does not stick to forced B3 (or any other B*) and roll back to B20 - the band is just not available or signal to weak
If there is no entries in the log - router didnt even "grip" on any other band/cell.
Me trying to force-connect to other cell(on different mast) few years back - roll-back as signal was not strong/steady enough

R WiFi(other) - idea the same, different setup/equipment/control.
It depends - some dont care/dont want to care, as long there is internet… until when there isn't and you stuck.

Your neighbor is in contract(as everywhere - T's&C's/caps/FUP) for at least a year and cant terminate/switch without fees, service still depend on third parties like mobile providers, etc. Controls might be limited as they do not owe gear.

With unlocked router i have options to chose ISP and switch at will, with PAYG plans available dont have to "lock" myself into contract and i control/manage the equipment i owe. Suite me, might not work for everyone.

Your situation is what it is and it will take you to decide - R WiFi/satellite/other - whatever works best to get result that is best at your circumstance. Investment in to gear is risk, especially where you have no assurance of outcome.
This why i suggest to investigate nearest sites for band availability (cellmapper/other) before commitment. 2km from mast is not big factor - i was running ~250/50 for several years from mast 4km away(external antenna, B1/3/20/28 available on the mast). ISP change their gear on masts too(might be Eir "killed" B3 or fault ?) - once in the while might be worth to get 3/VF SIM for a change, again - unlocked router needed.
BTW , is you B628 locked? is it labeled with Eir/has Eir logo in the GUI(web interface) ? did you attempt to install 3/VF SIM into it ? after all it might be not locked - worth a shot to test(if you have someone with other provider phone ask them for a cup of tee/coffee/pint and …… )

not tracking T&C's on three currently so dont know if there is such thing, perhaps some new introduced. ISP's print those in amount we would not have enough a**** to wipe.

Always was under impression there is non with three, I use to do ~1TB for months and never had any issues/restrictions, but i am on "legacy" SIM, might be different

If you still have three SIM - stick-it-in , as long its still active (<180d since last top-up/use) even if you have no credits, you will see if it register on the network or fail

If its truly locked, i would not be good adviser how to unlock. I have done it to two devices and was in luck, but i had accepted the risk of "bricking" it before-hand and had "backup" device in place.
It takes some research and might involve paying third party to get unlock code or soft to generate one.
I think it works "cheaper"/less hassle to get unlocked device in first place, especially with sellers that can be trusted with warranty/return (like CEX), unless you into it…