He was a descendant of an old ecclesiastical family from County Laois, who were the hereditary comarbai of Colum moccu Lofgse, sixth-century founder of the monastery of Tfr da Glas (or Terryglass) in County Tipperary, and friend of Colum Cille. Aed was one of the principal compilers and scribes of the great twelfth-century literary-historical compendium, the Book of Leinster, also known as Lebar na Nuachongbala, the Book of Oughavall, which was his family’s ancestral home. Much of the writing of the manuscript may have been completed there. He signs himself on 32r (p. 313): "Aed mac meic Crimthaind ro scrib in leborso 7 ra thinoil a llebraib imdaib" (Aed Ua Crimthaind wrote this topic and collected it from many books). He is not the finest scribe in the Book of Leinster, but he evidently played a key part in the compilation and redaction of the many texts which went into it. The identification of his hand in certain places is still a matter of some paleo-graphic difference, particularly in those entries referring to events post-dating 1166.
He is also the recipient of the earliest Irish personal letter, written to him by Finn mac Gormain, bishop of Kildare (d. 1160), copied into the tale known as Cath Maige Mucrama on 206v of the manuscript. It is the earliest vernacular example of the medieval ars dictaminis. It has the usual form of a rhetorical epistle, praising him for his learning as "chief historian of Leinster in wisdom and knowledge and book-lore, and science and learning." It requests that the tale Cath Maige, being dictated to his scribe by Finn, be completed by Aed, who apparently had access to a better or fuller copy. It concludes by asking him to send a copy of the duanaire of Mac Lonain, "so that we may study the meanings of the poems that are in it." The letter also styles him fer leigind (man of learning) to the king of Leth Moga, perhaps Diarmait Mac Murchada. Although there is no independent evidence for the assertion, it is often claimed that the famous references to the exile of Diarmait Mac Murchada, "king of Leinster and the Foreigners" in 1166 and to his death in 1171, at the end of the prose regnal list of Leinster (f.39d)—Saxain. iar sain miserabiliter regnant Amen. (And, after that, the Saxons miserably reign)— indicate a close personal relationship between Aed and Diarmait. His span of scribal activity on the manuscript can be judged from the date of his first personal entry, which records the death of Domnall Ua Conchobair in 1161. Best identifies his last entry in the notice of the death in 1201 of Ruaidn mac Con Ulad, but it is very doubtful that an individual who was fer leigind of Leth Moga in the middle of the twelfth century was still alive in 1201.
