Dont know where exactly you are Damo but I'd suggest the National Museum at Collins Barracks, Dublin.
We finally got round to visiting it and had a thoroughly interesting couple hours meandering through the exhibits. The building itself is interesting too. In the courtyard, there's a replica of a Viking Longboat on display for a short time.
Looking at the website, I see there's a family tour on March 16th and a special on St Patrick's celebrations on March 18th.
Oh yeah, and it's free entrance!