these structures are known as Martello Towers..they were built in strategic locations throughout the British Empire in the 1800's
and used as observation posts by the british Army ,this one here was occupied by the British in WW1.
Durning WW2 it was occupied by the Irish Police force
Today it is the Museum of Vintage Radio
it is located on top the hill at Howth
here is a link en.wikipedia.org/wiki/Martello_tower