Dún Ailinne (]; sometimes anglicized Dun Aulin) is an ancient ceremonial site on the hill of Cnoc Ailinne (Knockaulin) in County Kildare, Ireland. It is southwest of Kilcullen, near the N78 road to Athy. It is a large circular enclosure covering most of the hilltop; about 13 hectares. While there is no longer anything visible on the ground surface, archeological investigations show that there were once circular timber structures (or rings of upright timbers) inside the enclosure during the Irish Iron Age (ca 400 BCE - 400 CE).