Deda mac Sin (Deda, son of Sen) was a prehistoric king of the Érainn of Ireland, possibly of the 1st century BC. Variant forms or spellings include Dedu, Dedad, and Dega. He is the eponymous ancestor of the Clanna Dedad, and may also have been a King of Munster. The generations preceding Dedu mac Sin in the extant pedigrees appear artificial. Eventually they lead through Ailill Érann to a descent from Óengus Tuirmech Temrach and thus a distant kinship with the Connachta and Uí Néill, whose own pedigree is in fact unreliable before Túathal Techtmar. Dui Dallta Dedad was a foster-son of Dedu.