I am indebted to Redmond O'Brien - how much more Wexfordian can you get? - for alerting me to the unusual green buoy marking the danger of the Ballast Bank situated in Wexford harbour. For some reason, this light does not appear on Trabas, one of the very few omissions I have come across in that wonderful resource.
In fact, I have found nothing online about the light. Regarding the Ballast Bank, itself, it is an artificial island, constructed so that ships might pick up or discharge ballast on entering or leaving Wexford Harbour. Most sources give the date of construction as 1937, though the architecture of Ireland site - which really should have the inside track on these sort of things - dates the island back to 1831. I'm no expert, but the light itself seems older than 1937. Someone should bring it in to the Antiques Roadshow.
Below, a drone's eye view of the island, which I filched from Wexford Hub, an excellent site about all things Wexford.