The FAI Cup is a tournament that was created in 1922 and in which 40 football teams participate. It is played annually and whoever wins gets access to the first qualifying round of the UEFA European Conference League. So far, the top winner is Shamrock Rovers. 
If you are in Ireland, buy your tickets to see a FAI Cup match and live the experience with a local fan who will make you enjoy the preview, the match and the post-match as if you were one of the fans.