I've been to Dublin several times and, in fact, will return in September. While I am hardly an expert I agree that drinking a Guinness is as good as visiting the factory. In my view the Book of Kells at Trinity College is worth multiple visits. Personally, I think the Dublin Castle tour is quite interesting. I fully intend to visit the Kilmainham Gaol and ashamed to say I have not yet been there. And listen to music any night at the Brazen Head.
The bus from Dublin to Galway, and then the connection to the Aran Islands, is easy breezy and a satisfying way to go. The boat over can be great adventure. I've been to Inis Meain twice (Rick doesn't cover it as Inis Mor gets all the glory) and I hesitate to tell anyone how wonderful it is, but if you want total solitude, a bit of walking, an island that you can almost memorize in 2-3 days, dun fortifications (two, in fact), thatched houses and authenticity (all signs are in Gaelic except the ones saying "keep out" or "private"), then Inis Meain qualifies. Lean over the cliff and gasp at the North Atlantic. If you fall over, no one will know. Talk about isolation. If Maire Mulkerrin is still alive, stay at her B&B. She is iconic. But you have to walk to get anywhere. We are talking small and isolated. And lots of limestone.
Trim Castle is worth a visit in my view. It's one of the best preserved Norman castles still in existence in Ireland. And Bru na Boinne/Newgrange is a wonderful day trip. Fabulous history. Take the tour. You can take the bus out of Dublin to the north on an easy day trip to each (one day for each, not in the same day). County Meath is the epicenter of so much ancient Megalithic/Gaelic history and it's wonderfully close to Dublin, just a bus ride away.