Hi David
As a follow up to Brendan's post, PTSB now state on their website that "you can also apply for a tracker portability mortgage if you moved to a rented property and are renting out the home you own." Not sure if adding an additional borrower (I assume the mortgage on the apartment is in your sole name) complicates things - I wouldn't have thought so.
The other option, of course, is to continue renting out the apartment (although it might restrict the amount you can borrow for your new home). It's obviously a cheap tracker rate so whether or not it makes sense to retain the apartment as a rental (rather than porting the tracker to your new home) really turns on:- (a) the rent you are getting on your apartment and the expenses related to maintaining the property; (b) whether or not you have a diversified portfolio of other assets (e.g. a pension) and sufficient cash reserves to meet emergencies; (c) to what extent (if any) it will impact your ability to borrow for your new home and what interest rate you would be charged if you don't port the tracker; and (d) your general attitude to being a landlord (do you find it stressful?).
In the vast majority of cases, I think it makes more sense to opt for a tracker mover option over retaining an apartment as a rental.
Hope that helps.