 Ninety-nine folios of Giolla Isa's survive (which may be termed, for convenience, 'Leabhar Giolla Íosa' – LGÍ), containing some of the most important Irish literary texts from the Old and Middle Irish periods, including the only (virtually) complete copy of Rescension I of Táin Bó Cúailnge.Two colophons by Giolla Íosa indicate 1392 as the date of writing, though the work may not have been brought to completion for some years after that" 

Giolla Íosa was assisted by his student, Murchadh Ó Cuindlis.Ó Cuinnlis penned an excellent manuscript in east Ormond (now County Tipperary in 1398–99) which is now part of YBL.