Arkin's Castle is a ruined tower on the foreshore near Killeany Bay, the most prominent historical structure visible from the ferry pier. The name is anglicised from the Irish — the castle itself was built by Cromwellian forces in the 1650s and used as a military garrison to control access to the islands. The original medieval fortification on the site is older, but what stands is largely the Cromwellian reconstruction.
The ruin itself is modest — a roofless tower, its walls still standing to most of their original height. The setting does most of the work: grey stone against grey Atlantic sky, the ferry dock a few minutes walk away, the whole scene compact enough to take in at a glance. Worth ten minutes of your time on the way to or from the pier.