Liz Gabay wrote:

> LU   Is de sin luid Flidais la Fergus dochom a chríchi bunaid co  
> ngab rígi
> blogi do Ultaib .i. Mag Murthemni cosinni baí i lláim Con Culaind meic
> Sualtaim.

Is de sin = It is of that = For that reason

luid Flidais = Flidais went

la Fergus = with Fergus

dochum a chríchi bunaid = to his territory of origin ("dochum" takes  
the genitive)

co ngab = so that / until (s)he took

rígi blogi do Ultaib = the kingship of a portion of Ulster

.i. Mag Murthemni  = that is, MM

cosinni = with the thing

baí = (that) was

i lláim Con Culaind meic Sualtaim = in the hand of CCmS

I'm not sure what the last bit, "what was in CC's hand" refers to.