Just an update to add to the recommendation of the free walking tour of Dublin, starting at 11am daily from City Hall (tips to guide if you like it).
Some Australian visitors found it very good indeed, with amusing, informative and entertaining guide. Definitely on the 'to be recommended list'.
They also visited Croke Park and enjoyed the tour.
I took them to Newgrange one day, and we did the tours of both Newgrange and Knowth - well worth €11 per head. 3 hours for the 2 tours and the time flew. Bring warm clothing, as its very exposed.