No those are not UFOs descending on Limerick, they are frisbees. Limerick makes itself home to The Siege of Limerick, one of the largest ultimate tournaments in Ireland. The event is competitive while friendly at the same time. Even if you don't play ultimate you can come for the great parties that are known to accompany such tournaments.
Disclaimer: Event details may change at any time. Please review the official website or check with the
event organizer when planning to attend the event.