well, it depends, the best return may not be on a propery in the "best area" - ie if you can buy a cheap house out in D15 and be certain to rent it out that might be the best place for a good return.
then again, there's a good chance of there being an over supply of properties in that area so you could risk not renting the place out.
an area that students rent in is possibly good but then again, they're likely to wreck the gaff and it's likely to be empty every summer and short-term leasing is very difficult.
why does your father want to buy in dublin? if he's not going to be an active landlord then he'll have to use an agency to manage the rental which is expensive, if he is going to manage the rental himself how quickly can he get to dublin to fix things?
how much does he want to spend on the property? that will probably narrow the areas down a lot.
