I went to Dublin for a Hen's Night in 2003.
A bus service ran around the city and went to every major attraction. Limerick Castle, Guiness Museum, military monument I can't remember the name of, the cathedral, among other places. The cost for an all day ticket was 10 Euros. The service was prompt and very easy to use, not to mention some of the drivers were hysterically funny. It was a bargain, and we didn't have to worry about parking.
I was hesistant about spending time at the Guiness museum, but it was much better than I thought it would be. When you buy your admission pass you automatically get a free coupon for a pint of Guiness, to be enjoyed at the Gravity (?) Bar. It's the museum's circular bar that offers a 360 degree view of the city, and that alone is worth visiting Guiness Land.
A word of caution. Before I visited Dublin I had no idea bartenders tended to ask ladies if they wanted a squirt of Ribena in their pint. My reaction then and now was ewwww ewww and double ewwwwwww.
Have a great time!