GF moving into apartment

if you want to claim rent-a-room for your OH living in your place, then you'll have to sleep in separate rooms to be eligible! If you are both sharing a bed(room), then obviously she isnt renting a room of her own to sleep in.

Beware of all those Revenue/SW catch all rules which, in line with the Constitution, mean co-habiting couples cannot be in a more favourable position than married. Married cohabiting couples cant claim rent-a-room for each other, so it follows that non-married cohabiting couples definately cant.
 

But if the government won't recognise a couple living together as a common-law marriage or anything of that sort, then surely all that's left is 'roommates'?

Also, the owner won't actually gain anything by renting the room more than he would if they were married. The partner however will be in a lesser position, since if they got married she would be entitled to 1/2 of the house. And if the owner can't avail of the rent a room scheme, then he will also be at a significant disadvantage, and have to pay tax on money the partner pays towards the mortgage.

All that is said for discussion/debate, and I agree that the OP should call and ask the revenue. And please, report back what you find out!