First I'd like to say how much I love the Irish. They're incredibly nice, funny and good storytellers!
I stayed at Isaac's for a week. The staff at the reception is great; very, very friendly, helpful and fun to talk with (Irish huh). Just to meet them I'd recommend this hostel :)
Isaac's is clean and well-maintained: the facilities are cleaned every day. Breakfast is simple, but good enough for me (come on, it's free!).
It does get very hot in the hostel and in the room (obv I can only speak for the room I slept in). I didn't sleep very well due to inconsiderate neighbours and the noisiness of the room. The floors creak badly when someone walks around and the beds are even worse. For a 16-bed room there are way too few wall sockets (I counted four. Maybe they have extension cords at the reception, didn't ask). There is one small-ish window, so the room gets very hot very fast when 16 people are sleeping there. The fan runs on electricity, and because everyone charges their phone at night it was always unplugged.
Maybe I couldn't sleep like this, but many others can (my friend, for example, slept like a baby). People like me can use earplugs (€1 at the reception) and an eye mask, which many hostel-goers do anyway. With those and maybe an XXXL-coldpack, even the lightest sleeper will be fine!
So overall; if you just want a place to stay at a great location, some good advice about your trip and don't expect a 5-star hotel, you'll definitely enjoy staying at Isaac's!