If you look over the last couple of years posts on similar questions, and on posts concerned with letting property ,the general opinion which i strongly share is..
... absolutely do not do the two property idea -i.e. living in one whilst letting the other.
If you are able, with lender's permission, to sell your NE house and ,with savings and a loan ,obtain a bigger home then that is what you should do.
Letting is not as profitable as people think it is. It can be a risky business. And the govnt is not making it easier continually adding one tax,fee or charge and another -plus a whole host of regulations.
Nobody knows if the 75% relief will go or diminish, but you must take this into your worst case scenario - along with not finding good tenants or tenants not paying or causing damage.
Considering you're starting a family you really don't need the hassle and stress for what could be an almost profitless argument.
If you can get the O.K. to buy a bigger house then do it now and get rid of the one you're in.
P.S. You could get up to €10k p.a. tax-free if you rent-a-room. It doesn't have to be only one room for which,of course, you wouldn't get 10k. But if ,when house-hunting, you find a house that has an almost self-contained room or two you could make far more letting this on tax-free basis than letting a whole house with the tax liabilities.
And when the kid/s grow up you can kick out the lodger and it's more space for you.
Read about it on the revenue.ie website.