Mac Crimthainn, Áed (d. p.1201), is the only known scribe of the Book of Leinster. His genealogy (identified by F. J. Byrne) occurs in his own hand in the manuscript (p. 313). The relevant parts, back to the eponym of the surname, read: AEd m Gillai Ciarain . . ./comarba Coluim/m Gillai Fhinneain m Cinaeda m Crimthaind (Áed son of Gilla Ciaráin (the coarb of Colum) son of Gilla Fhinneáin son of Cináed son of Crimthann). The scribe identifies himself on p. 313: Aed mac meic Crimthaind ro scrib in leborso 7 ra thinol a llebraib imdaib (Áed son of Mac Crimthainn who wrote this book and compiled it from many books). R. I. Best (qv) interpreted mac meic as the ua (grandson) element of a surname and read the scribe's name as ‘Áed Húa Crimthaind’; but the scribal form is clearly to be read as Aed mac Meic Crimthaind and is to be understood as showing that Áed's father Gilla Ciaráin was still alive and was head of his name. The title comarba Coluim [meic Crimthainn] ‘the coarb of Colum [mac Crimthainn]’ refers to the headship of the monastery of Tír Dá Glas (Terryglass, Co. Tipperary), founded by Colum of the moccu Cremthainn Áin (d. c.550). Áed's surname Mac Crimthainn also occurs in a transcript (p. 288, marg. inf.) of a letter from Find Ua Gormáin (qv) (d. 1160), bishop of Kildare, to Áed: Betha 7 slainte o Fhind epscop (.i. Cilli Dara) do Aed mac Crimthaind do fhir leigind ardrig Leithi Moga . . . 7 do chomarbu Choluim meic Crimthaind 7 do phríms[h]enchaid Laigen (Life and health from Finn, bishop [i.e. of Kildare], to Áed Mac Crimthainn, lector of the high-king of Leth Moga . . . and coarb of Colum mac Crimthainn, and chief historian of Leinster).
Despite the variety and inequality of the script Best believed that the manuscript was the work of a single scribe, written over many years, but William O'Sullivan (1921–2000) claimed it to be the product of a scribal school. Tír Dá Glas in Lower Ormond was in the Munster sphere of influence, but two notes in Áed's hand, referring to contemporary affairs, immediately associate the manuscript with Leinster: (1) a reference (p. 49) to the killing (indé ‘yesterday’) of Domnall son of Congalach Ua Conchobair Fhailge (1161), and (2) a note lamenting the banishment (1166) on that very day (indiu (.i. Kl Aug.)), of Diarmait son of Donnchad Mac Murchada (Diarmait Mac Murchada (qv)), king of Leinster (ri Laigen). It would seem that ardrí Leithi Moga (high-king of the Southern Half), Áed's patron (as referred to by Bishop Finn), was none other than Diarmait. Áed Mac Crimthainn probably lived in his ancestral territory of Uí Chrimthainn Áin in Laígis at An Núachongbáil (Oughaval, near Stradbally, Co. Laois), a location that gave the manuscript its medieval and first known title, ‘Lebar na Núachongbála’ (The Book of Oughaval). An Núachongbáil was situated close to the fortress of Dún Másc (Dunamase; the ‘-e’ was originally a scribal error), owned by Diarmait Mac Murchada but later handed over by him to his son-in-law, Strongbow (Richard de Clare (qv)).
The manuscript contains some unique texts of Leinster interest, but (because of the Mac Crimthainn connection with Tír Dá Glas) scholars have long been referring to it as ‘the so-called Book of Leinster’. The reference in Bishop Finn's letter to Áed as coarb of Colum mac Crimthainn may only mean that he was the holder of this ancestral titular office. Many of the manuscript's texts have been marred by misguided scribal emendation which may largely be attributed to Áed, but this major question has not been addressed in relation to the possible number of primary hands in the manuscript.