Ok, just to make it clear once for all: I'm not talking about the Aran IslandS, the ones in front of Galway, but the Aran Island (no S you see?! It's just one island) also known as Arranmore, in County Donegal.
Anyway, the Aran Island, which is, according to Wikipedia, "the largest inhabited island of County Donegal, and the second largest in all of Ireland". It is a pretty big Island, or at least for irish standards, and if you get there be sure you are with a car or at least with a bike, unless you really, really like to walk.
We visited this island first thing in the morning of the second day of our trip to Donegal, and I have to say that is a nice place to visit, but I wouldn't spend more than 3/4 hours in there, as is big but not that big, especially if you are driving.