>> Feachtus tra do Diarmaid mac Cerbaill oc tomailt fheisi na Temrach, Mugain  ingen 
Concraid meic Duach di Eoganacht Chaisil fora laim .i. mathair Aeda meic Diarmada. 

Thanks, Neil,
  I have 
Then once Diarmaid (was) eating (at) the feast of Tara, Mugain the daughter of Conchrad 
mac Duach of (the) Caisel Eoganacht at his hand, the mother of Aed mac Diarmaid. 

feachtus -- variant of 'fechtas..journey, expedition...turn, time, occasion...once'
tra-  looks like 'trá...then, so, indeed...however'
tomailt -- verbal noun of 'do-meil...wearing away, consuming..act of consuming food or 
feis -- verbal noun of do-foaid...spending the night, sleeping..cohabitation...entertainment 
for the night, food, supper ...a feast, festival

Comments welcome, Liz

>There is a parallel passage in Egerton 1782, for which I think we have to
>rely on O'Grady's transcription in Silva Gadelica at page 74.25f. He has:
>Fechtas do Diarmait i Temraig oc fledugud . Mugain ingen Concraidh mheic
>Duach do Eoghanacht chaissil for a láim .i. máthair Aedha mheic Diarmata .