'A' rated house but gaping vent holes

You can put baffles in them to prevent wind pushing in and some noise. But do not block them up. Your house needs to breathe. Ultimate solution is to install MHRV or DCV system in the house in replacement of the fixed vents.

Fixed vents are very rudimentary but they do a job.

The wall vents are to provide background ventilation and air changes to reduce moisture in the air in prevent damp/condensation. All houses require ventilation, even A rated houses. In fact especially A rated houses, as they're generally more insulated and airtight (with regards the building fabric) and therefore need some form of ventilation. The trickle vents in the windows likely wouldn't be enough to provide this, so by insulating or blocking up the vents, you're increasing the risk of damp/mould.

Your best bet is to install something like these at the existing wall vents

They'll reduce noise/heat loss, and open/close depending on built-in humidity sensors.

Retrofitting Ireland. It's in 2nd half of programme.

Good luck wading through the ads

The building regulations insist on these holes when there is not mechanical ventilation. It is a terrible rule as windows are a far better way of providing ventilation an ventilation needs vary by household size. Why not let windows ventilate, it is their purpose. Id really encourage you going down the mechanical ventilation route. I have it and its wonderful. You could also get your BER report which will be in the SEAI database, done by the builders and see what your houses airtightness is.

Does anyone here have experience fitting decentralised heat recovery in hole in the wall houses? Do you need one in every room where there is a vent?

The Aereco vents you linked to can only be installed as part of their full DCV ventilation system. I've yet to come across a similar product that can be easily retrofitted into existing vents. Like the OP I have first hand experience with these A rated homes and the hole in the wall vents. They are especially prone to over ventilation when it is anyway windy. It is even worse when paired with a heat pump (so typically low intensity constant heat which struggles with sudden cold bursts) and it often means the room feels a lot colder than it should be.

A MHRV is the obvious solution but a retrofit is a big job to get the ducting in place and not likely feasibly for someone who has just moved into a new build.

It is foolish way to cut costs but when I am driving around new suburbs of Dublin I get the impression that it is common. I can't always tell from outside if they have hole in the wall vents or decentralised mechanical ventilation but sometimes you can.

Someone should do a study of energy bills of post 2011 homes with hole in the wall vents and heat pumps.

was that the unit?

No building is air tight . There is always some level of infiltration, you can't as a blanket statement say that buildings built across 2 decades have enough infiltration to allow for sufficent ventilation.

Monitor the CO2 in an average bedroom built in any decade with windows closed and no dedicated ventilation and you will quickly find the level of infiltration is completely insufficient to maintain co2 or moisture levels at an ideal level.

Very good point but it is possible to have a reasonable airtightness value (and thus poor ventilation) but also also have a freezing house, due to the plasterboard tent effect. The risk of this is particularly high in windy areas like Ireland. I dont know if 60-80s houses have the plasterboard tent issue but in the 1990's you would. I don't have any data on this but I have experienced this.

In my limited experience I've only seen dot and dab on party walls. Everywhere else I've only every seen plasterboard on timber studs. That's from the late 70s onwards. Might have just been the people I worked with. Might be more common in apartments and one offs. I have experienced that tent effect on those walls with studs and plasterboard on houses built in the 2000s. You might feel a draft under a skiting board or closed vents. You might also see dust blown away from the skirting board. But thats probably due to poor construction and patchy insulation.

There was a study from the SEAI about poor construction standards in the 90's, 2000s, (I think) where it showed the poor insulation and draughts of those houses, about 90% weren't up to the standard. But I can't find it now. I would say peak Celtic tiger, there was a lot of poor quality building going on. Things were just thrown up.

Other people will more experience would have a more informed and experienced viewpoint.

Is it possible, or should I say legal, to DIY a MHRV install or does it need to be certified etc?

Any recommendations? (brands etc)

2000+ sq/ft dormer

Block those vents up, it's all bull. There is enough ventilation with the coming and going of people, the opening and shutting of windows and doors, Geez.

It's easy vent your home on a warm day, just open the windows, having vents like that open on a frosty night is bull.

Totally again it's awful advice, I have an older house, currently looking vat getting vents installed due to ventilation and damp issues...