From the Garda website
A member of An Garda Síochána may seize a vehicle being used in a public place where –
•A member of An Garda Síochána is of the opinion that the vehicle is being used in contravention of Section 56 Road Traffic Act 1961 i.e. The person driving the vehicle is not insured to drive the concerned vehicle.
•The person driving the vehicle refuses or fails to produce there and then a driving licence then having effect and licensing him/her to drive the vehicle, when production of such a licence is demanded of him/her by a member of An Garda Síochána under Section 40(1) of the Road Traffic Act 1961 and the member is of the opinion that the person is by reason of his/her age ineligible to hold a driving licence, licensing him/her to drive the vehicle.
•There is no road tax in force for a period of 2 months or greater.
Reality is that Gardaí are unlikely to seize a car for non-production of a licence. However if you have no NCT and they are satisfied that the car is not roadworthy, they might seize it. Likewise, I wonder if you have no NCT are you potentially invalidating your insurance.