Adding names to a property deed

This is more complex than you might envisage.

You are effectively proposing to gift a share of the house to your cousin and that will require a deed of transfer to be drawn up. There are different ways in which his share could exist and that would need to be carefully considered, both in terms of the present scenario but also looking to the future.

There are significant tax implications too - Stamp duty and capital acquisitions tax on his share for a start. If your house was worth €300,000 and you cousin was to acquire 1/3 of it. The CAT liability alone would be €26,500.

There a likely better ways to structure whatever it is that you wish to achieve, and I would suggest getting robust legal/tax advice before doing anything.