Spanish definitely can be useful, and I wouldn't be surprised if the others were, especially in Dublin. That said, I suggest you get out of Dublin and visit other towns. The Cliffs of Moher are truly a spectacular sight, regardless of the weather. The town of Doolin is also right beside it, and it hasn't earned its reputation as the Mecca of Irish traditional music for no reason!
I personally also suggest visiting Galway, especially if you're there during the summer. It's a very artsy town, and I feel you get a more 'authentic' feel of Ireland there (or anywhere outside Dublin, really). You're also only a short boatride away from the Aran Islands, one of the most spectacular locations in Ireland.
If you wish to stay in/near Dublin, there is a relatively famous pub, O'Neill's, that does live Irish 'trad'. But the trad here is definitely not the same trad you'll get in Doolin; it's more along the lines of the lyrical works of the Dubliners and such. And Galway Girl will likely be played at least once. It's also part of the pub crawl most hostels offer, but it's still a nice place to listen to some music and get some food.
Another pub is the Brazen Head. It's the oldest pub in Ireland, and, when I was last there, even had a story nights and such, which were interesting. It's also said to be haunted by the ghost of the Irish revolutionary Robert Emmet (known for his famous self-given eulogy) One thing you must do at any pub you go to is drink the Guinness, though. It's got a much better taste than it ever can in the States.
And speaking of Guinness, there is both the Guinness Storehouse and the Jameson Distillery which are open for tours. Both are quite interesting to see, and you can even pour your own pint at the Storehouse.
Trinity College is also a place you need to visit. It is a beautiful campus and its library is famous for being one of the most beautiful libraries in the world. And the Book of Kells is housed there, so there's a possibility you can see it (sadly, I don't know the rules as I went as a guest of a student when I saw it). There's also
Glendalough in the Wicklow mountains right outside Dublin. It's the area where St. Kevin lived, and where he famously kept his arm outstretched for the blackbird. It's got breathtaking scenery, and is an absolutely wonderful location, especially if you're there on a clear day. A walk down Grafton street is a must too.
And, if you don't mind reading, you could try reading
The Dubliners or
Ulysses beforehand and visiting some of the places mentioned in them to see how it's changed. There's also a "viking tour' that goes through Dublin. It gets kinda annoying to the locals, but it's great craic for those on the tour!
Kilmainham Gaol is a former prison turned museum that has a lot of connection to Irish history; could be interesting.
You can also visit Newgrange, which is in close proximity to Dublin; it's a neolithic tomb that is pretty famous. And, while out there, you can visit the Hill of Tara, which is where the High King of Ireland was traditionally crowned. Also outside Dublin there's Waterford Crystal, which I believe offers tours as well. Could be something interesting. Killarney is also an amazing place to visit, as is the entire Dingle peninsula. If you go to Killarney, make sure to visit the Killarney National Park; the Muckross House and the surrounding areas are some of the most beautiful landscape in the country.
There's some famous bookstores, but you really won't find anything in them outside the ordinary, as far as I can recall.