Overall, Frank has a great place in the heart of Dublin. It's within walking distance of almost anything worth seeing in town, and is very close to some of my favorite places to eat and drink.
The location, as others have mentioned, is near an adult shop and a sex club. The entrance to the apartment is not indicated well, but it's in the alley with the sex club entrance. They also have a large mural depicting some kink/fetish scenes. It wasn't a problem for us, but it may be for you. Also, while we didn't have any problems, the entrance in the alley can be a little sketchy.
The apartment is pretty nice, with two bedrooms, a nice bathroom, a kitchen, and a combined living/dining space. Unfortunately, the hot water needs to be turned on manually, and any time the water is running, there's quite a bit of noise that sounds like a water pump. It can make it hard to sleep while others are showering (or even just using the sink in the bathroom or kitchen). This, for us, was the largest downside to Frank's listing.
That said, for the two nights we were there, it was a pleasant place to stay and kept us near all the things we wanted to see.