Easier to wait until you are married. It will be cheaper. It is not that terribly convenient to add a name to a mortgage (you have to involve the lender in the paperwork to a greater degree). However, nor does it particularly matter whether the mortgage is in joint names; What usually happens is that people just leave it until perhaps they are remortgaging, and the new mortgage will be in joint names as a matter of course;
It is quick and relatively cheap to put the family home of a married couple into joint names. This is a commonplace transaction and your solicitor will probably do it for as little as €250 plus VAT (many solicitors deliberately charge very little for this because - rather like with wills - they want to encourage people to do it)