Deda mac Sin 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. Through his sons Íar mac Dedad and Dáire mac Dedad, Dedu is an ancestor of many famous figures from legendary Ireland, including his "grandsons" Cú Roí mac Dáire and Eterscél, "great-grandsons" Conaire Mór and Lugaid mac Con Roí, and more distant descendant Conaire Cóem. A third son was Conganchnes mac Dedad. Through these Dedu is also an ancestor of several historical peoples of both Ireland and Scotland, including the Dál Riata, Dal Fiatach, Múscraige, Corcu Duibne, and Corcu Baiscind, all said to belong to the Érainn, of whom the Clanna Dedad appear to have been a principal royal sept. 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. A proto-historical sept of the Clanna Dedad are known as the Dáirine, descending from Dáire mac Dedad and/or Dáire Doimthech, and are later known as the Corcu Loígde. Alternatively this may be used synonymously, with some confusion created by their identification with the Darini of prehistoric Ulster. In any case, the Darini and Iverni are clearly related. According to the Book of Glendalough and Laud 610 pedigrees, a brother of Dedu was Eochaid/Echdach mac Sin, from whom descend the Dál Fiatach of Ulster. But alternatively they descend directly from Cú Roí mac Dáire, and thus from the Clanna Dedad proper. The precise relation of the Dál Fiatach to the Ulaid of the Ulster Cycle, rivals of the Clanna Dedad, is lost to history. Eoin MacNeill finds the Conaille Muirtheimne to also descend from Dedu mac Sin, from another son Conall Anglonnach, believing they are quite mistakenly thought to be Cruthin, as found in "later" genealogies. Dui Dallta Dedad was a foster-son of Dedu. There is also an Ogham of Dedu found in the Book of Ogams. Over one third of all Irish ogham inscriptions are found in the lands of his descendants the Corcu Duibne.
The Síl Conairi were those septs of the Clanna Dedad descended from Conaire Mór, namely the Dál Riata, Múscraige, Corcu Duibne, and Corcu Baiscinn. The first, presumably settling in far northeastern Ulster in the prehistoric period, would famously go on to found the Kingdom of Scotland. The Royal Family of Scotland, the House of Dunkeld, were described as the "seed of Conaire Mór" as late as the twelfth century. Through the House of Dunkeld and Conaire Mór, Dedu mac Sin is an ancestor of the modern British royal family. The last king in the direct male line from the Clanna Dedad and Sil Conairi was Alexander III of Scotland. The remaining Síl Conaire would settle and/or remain in Munster, where, although retaining their distinctive identity, they would be overshadowed first by their Dáirine kinsmen, and later fall under the sovereignty of the Eóganachta. But it appears the Síl Conaire, and especially the Múscraige, actually acted as prominent facilitators for the latter, and this would presumably have been in opposition to the Dáirine. A late and unexpected king of Munster from the Múscraige was Flaithbertach mac Inmainén. The birth, life, and fall of Conaire Mór are recounted in the epic tale Togail Bruidne Dá Derga. Two distantly related tales of more interest to genealogists are De Síl Chonairi Móir and De Maccaib Conaire. In these he is confused with his descendant Conaire Cóem.
The Dál Fiatach and Cú Roí
The descent of the Dál Fiatach princes of Ulster from Dedu mac Sin is less secure, but nonetheless is supported by independent medieval sources.
The Dáirine (Corcu Loígde)
As early as 1849, the great Irish scholar John O'Donovan noted that the pedigree of the Corcu Loígde, the leading historical descendants of the Dáirine, is corrupt for many of the generations preceding the legendary monarch Lugaid Mac Con.
