It depends on what kind of rental market you're looking at. In Galway, if you plan on renting to students, houses in the newcastle / renmore /ballybane area are probably your best bets, or purpose-built student apartments with tax relief.
Otherwise, houses in more mature areas like Renmore & Salthill are good if you plan on renting to families.
You can reap a relatively high yield from renting modern vity-centre apartments to young professionals, but in my experience, turnover (and therefore hassle) tends to be relatively high....