Mag Eochagáinn was a direct Geoghegan descendant of Niall of the Nine Hostages – specifically from the branch of Fiachu mac Néill.
The Annals of Clonmacnoise is also known as "Mageoghagan’s Book" in honor of MacGeoghegan's translating this history into Elizabethan English and preserving its memory.
Mag Eochagáin dedicated this translation to his brother-in-law, Toirdhealbhach Mac Cochláin (Terrence Coughlin), whose family was among the last to uphold and practice native Gaelic Irish customs.
[6] The original manuscript of MacGeoghegan's translation is lost, but there are several copies of it in both the Library of Trinity College and in the British Museum.
Mag Eochagáin more than once refers "to the ould Irish book out of which he wrote, to the old Irish book which he translates, out of which many leaves were lost or stolen.." MacGeoghegan seems to have preserved the value of the original Gaelic phraseology and rendered it every justice as far as we can determine in the absence of the original manuscript.