Coarse Species.

Most of the species here today were brought by mankind and established themselves with self sustaining populations.

Irish native species are all salt water or brackish water tolerant are all cold water species: brown trout, the migratory and estuarine forms or brown trout, arctic char, ferox trout, pollan, salmon, sticklebacks, eels.
The warm water species (coarse fish) arrived later on.

The native species got here while Ireland was at the edge of the ice age ice cap, and swam around the fresh or brackish water at the edge of the ice cap, penetrating into meltwater streams that were emerging as the land ice cover melted. Char and ferox trout are up in mountain lakes because the sea was higher, the land lower with weight of mile thicknesses of ice on it, and much of Ireland was covered by temporary but vast province size lakes of melted ice. They swam into those mouintain lakes from the sea.
So the separate water catchments we know today did not exist at that time and waters mingled in the huge flooded areas.

Shad sturgeon burbot and smelt seem to have come later by sea migration. Of these, only shad and smelt remain.

Sometime after this Ireland separated from the UK and Europe land mass.

Pike arrived between 500 and 1500 years ago possibly introduced by bronze age man, possibly by alternative natural means.

Rudd, bream, gudgeon and minnow came some time after that.
Perch and tench came with the monks for monastery pond food species.
Roach came with the English planters to three waters and stayed in those waters. In the 1970s and 1980s roach were spread everywhere by match anglers and pike livebaiters.
Carp came in several introductions, and carp and tench were placed in suitable waters by the Inland Fishery trust in the 1970s and 1980s.
Dace, chub are being spread by amateur biologist anglers right now and are penetrating the watersheds they are already introduced into.
Zebra mussels came in imported boats and are being spread around to other watersheds by trailered boats.
The mitten crabs and chinese clams in the Barrow are recent, and are still penetrating the Barrow, Nore, Suir so far.

There is no uncontestable proof of when pike arrived, and probably never will be. We do know from old language, and the derivation of early Gaelic words that they predate or coincide with early Gaelic writings.
They are not a salmonid, and not salt water tolerant, and do not inhabit high altitude waters so they did arrive at the same time as the salmonids. Not being salt water tolerant suggests colonisation by landbridge, or by transport by man/other .. the proverbial "ducks legs"!
Pike are however still a cold water species, and it makes sense that they arrived soonest after, while conditions for reproduction were most favourable.

If you look at the characteristics of Irish freshwater fish - and eliminate the fish (fossil record) that were extirpated by ice ages prior to the most recent ice age ( we may have had tropical fish between ice ages) , the trend is for sea feeding fish that spawn in cold freshwater, then next cold water freshwater fish that spend all their lives in freshwater, then next warm water freshwater fish, colonising Ireland in that order according to climate.

You can also look at Irish fish morphology and colonisation by species in another way: governed by climate and temperature extremes that kill off non hardy species - originally Ireland was an arctic country (eg Greenland) then after that as a sub arctic country (eg Norway, Sweden, Canada) and later until now as a oceanic temperate country which it is today. In other words they came all the time, but only survived and remained here after moderated conditions permitted it.

Pike which you mentioned are also native to Canada, like Ireland, in the sense they are around longer than all other coarse (non coregonid pollan whitefish) species, and shorter than salmonids.
Did man bring pike to Ireland or did they pass along land bridges courtesy of big flash floods, land inundations from earthquake and tsunami, or landslides diverting rivers down the other side of mountains from their previous watershed? A lot happens in 40000 years, 2000 years, or 1500 years.

For example I think Ireland previously had zebra mussels and the last ice age extirpated them. I'm sure I read somewhere about fossil records of zebras.