They don't make as big a deal for St. Patricks Day as we do. It's more of a religious holiday over there. I think for major SPD activities, you need to go to Chicago or New York.
As far as Dublin, if you like Guiness, get ready for the best beer ever. I don't know why it is different, but the Guiness tastes a LOT better over there...and I love it over here. They are all about the Guiness. I remember asking some old locals which pubs to hit and the response was "This place pours a good pint of Guiness, and this place pours a good pint of Guiness...". Depending on your time schedule, I'd definitely suggest getting out into the country, especially the west coast. There's not a ton of stuff to do in Dublin outside of pub hopping. And the scenery is way better if you hit the road and travel a little.
Oh yeah, don't forget to visit the Brazen Head. Oldest pub in Dublin. Actually older than our country.