Martello Tower, Sandycove
A dozen or so of these were built when Ireland getting invaded by Napoleon. Napoleon opted for Russia instead. The towers are still here. We spent quite a while staring at the thing trying to work out how it was intended to serve as a fortification; it turns out that cannon was/were mounted in a swivel on the roof, mostly shielded by the walls, and not visible from below.
This particular tower is semi-formally called the "James Joyce Tower" because James Joyce stayed there briefly in 1904. Joyce allegedly overstayed his welcome and didn't pay rent, so his friend (Oliver Gogarty) contrived an incident where he fired a pistol into the wall over Joyce's drunken/sleeping self. The opening scene in Ulysses is set here, probably because of the event.