Fort Dunree is the best preserved and most dramatic of six forts built by the British on Lough Swilly following the 1798 uprising of the United Irishmen (which was supported by France), when fears of a French invasion were at fever pitch. The original fort, built in 1813, now houses a military museum, while the surrounding headland is littered with WWI and WWII remains you can explore. There are several good waymarked walks. Huge naval guns were added to the fort in the late 19th century, and during WWI the lough was used as a marshalling area for Atlantic convoys, and as an anchorage for the Royal Navy's Grand Fleet. Unusually, it remained in British hands after the partition of Ireland in 1922, and was only handed over to the Republic of Ireland in 1938.