This is an old Martello in Pormarnock Ireland that has been converted to a private home.
Martello towers were inspired by a round fortress, part of a larger Genoese defence system, at Mortella (Myrtle) Point in Corsica. The designer was Giovan Giacomo Paleari Fratino (el Fratin), and the tower was completed in 1565.
Great Britain and Ireland were united as a single political entity, the United Kingdom of Great Britain and Ireland, from 1801 to 1922, spanning the time during which most Martello towers were erected (the initial scheme started under the previous entities of the Kingdom of Great Britain and Kingdom of Ireland). Consequently, the Martello towers of Great Britain and Ireland can be considered to have been part of a single defensive system, designed to protect the coastlines of the two main islands of the British Isles as a whole.