Ballylin hillfort near Ardagh is the largest prehistoric monument in Co. Limerick. This paper presents the results of an archaeological investigation conducted there in 2012 by a research team from University College Cork. The surface features of the hillfort are presented by aerial LiDAR survey, with geophysical prospection also undertaken at the site. The defences, including one of the original entrances, were excavated to recover dating evidence, and to understand details of design and construction. Radiocarbon results suggest the hillfort was built in or around the twelfth century BC, during the transition from the Middle to Late Bronze Age in Ireland.