Commissioner of Irish Lights lists this as an unlighted beacon though the Lighthouse Directory says it was "probably not lighted until recently" The little ball on the top doesn't look like a light to me but I may be wrong. It was constructed in 1884 and marks the eastern headland of the narrow strait between Sherkin Island and the mainland at the entrance to Baltimore harbour. Barrack Point (next entry) can be seen in the above photo on the western side of the strait.
The beacon is the iconic emblem of Baltimore which incidentally has nothing to do with the slightly larger city in America. It is also known as Lots Wife. Wikipedia's Baltimore page says it was constructed in 1849.