Donnchadh Mac Ceallaigh (Donnchad Mac Ceallaig) or Donnchadh, son of Ceallach was a supposed 10thC King of Osraige. He is thought to have reigned as "Rí Osraige" from approximately 934 to 976 A.D.

Donnchadh was the son of Ceallach Mac Cearbhaill and the grandson of the famed Cearbhall mac Dúnlainge, whose career is dealt with in the Fragmentary Annals of Ireland text. This famous Cearbhall is a likely to be an ancestor of the Ua Cearbhaill sept of Osraige, whose lands bordered those of the Ua Donnchadha. The seemingly shared lineage between the Ua Donnchadha and Ua Cearbhaill septs of Osraige may be actual and is being further investigated by the O’Donoghue Society YDNA project.

Donnchadh is also reported to be the father of Giolla Padraig Mac Donnchadha, from whom it is highly likely that the famed Fitzpatrick (Mac Giolla Phadraig) sept sprung, though YDNA project investigations would lead us to question any such shared heritage between the Ua Donnchadha and Mac Giolla Phadraig septs.

Both the Annals of Ulster (where the one year adjustment has been applied) and the Annals of Tigernach indicate that Donnchadh Mac Ceallaigh died in 976 A.D. The less reliable 17thC Annals of the Four Masters maintain that he died in 974 A.D., though at this time many entries in these annals are thought to be two years in arrears.

Geoffery Keating’s "The History of Ireland" indicates that Donnchadh is buried in the grounds of the medieval church of Saighir Chiaráin (Seir Kieran, near Clareen, Birr, Co. Offaly). Keating further expresses that the king is buried in a mound to the west of the church and that his daughter Sadhb, married then to Donnchadh Donn Mac Flainn, King of Tara, is meant to have requested that a boundary marker, such as a wall, be constructed around church grounds at this time. Keating also writes that this church was the burial site for the Kings of Osraige during this period. It is interesting to note, that the Annals of Ulster record the death of the above mentioned Donnchadh Mac Flainn, King of Tara in 944 A.D., approximately thirty years prior to the death of Donnchadh Mac Ceallaigh of Osraige, whose burial, Keating reports, he attended.