I am coming late to your question, but just wanted to say...it is so great that you are interested in seeing some Gaelic Football (as we call it here in Ireland). Unfortunately, we've had very bad weather last week, and some GAA matches were postponed, so I'm not sure if the League Football Finals will still take place the weekend they were scheduled. They may have to push them back due to matches not having been played this past weekend.
Good websites to check: www.gaa.ie (GAA is the Gaelic Athletic Association, the governing body for all Gaelic sports); www.rte.ie (national broadcaster).
In relation to tickets for League finals, as well as ticket sales on the GAA website given above, there is also a supermarket chain called SuperValu, they sell tickets at many of their outlets. They are all over Ireland, but you can find a list of all the stores that sell GAA tickets here: www.supervalu.ie. Scroll down to the menu at the bottom of the page and select "GAA".
Of course, it is not all that difficult to see Gaelic football (or hurling - have you ever seen that?) outside of Croke Park, with no hassle getting tickets. Most local clubs have matches in the evenings or at weekends. You could easily stumble across one on your travels! The finals of the bigger competitions between the county teams (like the League and the Championship) are usually held in Croke Park, which is when it starts to get difficult to find tickets. But for the local matches between clubs, or even underage matches that children play in, you would have no difficulty with tickets and you would certainly get to enjoy some local colour!
Enjoy Ireland!