This is one of our rarest seabirds that has been declining over a number of years, though the species account on Wikipedia notes that the provision of nestboxes for the colony at Coquet Island, Northumberland led to a dramatic increase from 25 pairs in 1997 to an amazing 92 pairs in 2005! There are some great photos and video footage of this species at ARKive.
Caroline noticed from photos that the Tern had a ring on each leg. Following enquiries it transpires that this would be a feature of just about every Roseate Tern raised in the UK, Ireland & France. This bird is probably from one of the main colonies at Lady's Island Lake (Wexford), Rockabill (Dublin), Coquet Island (Northumberland), or our nearest colony in Brittany!