‘Domhnall Ó Gallchabhair’s Testimony’ is a treatise on the entitlements and rents due from the gentry of Tír Chonaill and Inis Eoghain to Ó Domhnaill (O’Donnell), the chief of Cineál Chonaill in the period preceding the Ulster Plantation of 1609. It is preserved in Cambridge Additional Manuscript 2766 (20) 7, folio 4 recto - 8 recto. The scribe does not sign his name. The English phrase ‘May 10th 1775 / received from’ is written on 6 verso thereby giving us a terminus ante quem for the text. The extant text appears to be a transcript of an original which must have been written down from the dictation of Ó Domhnaill’s stewart, Tadhg son of Tiobód Mac Loingsigh in 1626. The present article provides an edition of the text together with a translation and commentary. The historical context for the entitlements and rents due to Ó Domhnaill is set out in the introduction, which provides an overview of several similar texts from the North of Ireland, including Lebor na Cert (‘The Book of Rights’), Ceartl (‘The Entitlements of Ó Néill’), Cíos Mhic Mhathghamhna, and Buannacht Bhona í Dhomhnaill. Further contextualisation is provided by reference to relevant material written in Irish and English that was contained in a letter written by Sir Francis Shaen, an official of the English government in Ireland. This particular text provides us with a fascinating insight into Irish civilisation on the eve of the final phase of the English conquest of Ireland.