Hi Fergal, welcome to the club! I’ve just split your reply off into a separate thread so it’s easier to track. That’s a lovely car you’ve picked to build, and no doubt it’ll be great fun once you have it on the road. NSAI don’t have any centres up and running to do IVA as of yet, but the impression I got was that it wasn’t that far away. I’m overdue a conversation with them on the topic so I’ll definitely follow up. I hadn’t actually thought of the option for kit car manufacturers to organise IVAs in the UK before. British IVA is certainly recognised in Ireland, however there’s a chance it might confuse the tax office, as they’ll only be used to seeing it for already registered cars. Whether it’s here or there that the IVA is carried out, make sure you don’t register it in the UK first, as you’ll have to pay VRT there, and then when you “import” it to Ireland, you’d have to pay VAT and VRT. Sorry I can’t give a more definitive answer just yet, but I’ll do some digging for you!

Consider certification separately from registration, though you need certification in order to register.
Getting a UK IVA via Caterham sounds like a good idea, it’s certainly the path about which most is known.
The NSAI have an Irish IVA standard, and a manual but as Andrew says, currently no process by which you could go and get your car tested.
When I met with them in 2013, and again in 2016 after the Brexit poll, they said that a UK IVA would continue to be recognised here, though your form to ‘convert’ your UK IVA to an irish one will need additional information including corner weights not required over there.
The tricky bit is registration, and how much money you’ll have to pay, and to whom.

If money is no object, get your IE IVA (using your UK IVA), bring it down to an NCT centre like any other imported car, and pay what they ask for.
This will certainly involve VRT (% of OMSP multiplied by a factor based on the CO2 emissions of your engine, 34% for me). They have to establish what your car would sell for here on the open market, which is tricky and subjective. For me, I have a car that is 1-of-1 in the republic of ireland, and if they decide it’s a bit like an Ariel Atom and they want 34% of €70k, I have a real problem. At least with a Caterham you can point to previous sales. You should collect evidence of these, save screenshots etc.
The fun doesn’t stop there. If it’s a new build, new chassis etc, you might owe VAT of 23% on it too. And if you’re coming in from England, customs duty (12% I think).

https://www.revenue.ie/en/customs-traders-and-agents/brexit/brexit-for-individuals/importing-a-vehicle-from-great-britain-gb-or-northern-ireland.aspx
Importing a vehicle from Great Britain (GB)
From 1 January 2021 vehicled imported from Great Britain are liable to:

customs duty, if applicable
Vehicle Registration Tax (VRT)
VAT at the current standard rate.