>LU     Is de sin luid Flidais co Fergus mac Róich a comarli Ailella & 
> Medba fo dáig combad furtacht dóib ocon Tána na mbó a Cualngi. 

This is 3 days old so I'll post my notes.  Corrections welcome.   Liz 

     And because of that Flidais went with Fergus mac Róich on the 
suggestion of Ailill and Medb in order that there would be help for them 
at the Cattle Raid of Cualgne. 

de sin -- from that, because of that.  DIL D 12.5 lists this meaning 
of 'de' with the substantive verb rather than the copula.   So I suspect 
that 'is' may be  a form of 'ocus' here.

luid - 3rd singular preterite 'téit'

co - preposition 'with' used with dative.  I suspect 'Fergus' might be 
both accusative and dative;  the accusative sense of 'co' (to Fergus) is 
another possibility. 

a comarli --  see DIL C 341.80  which discusses "comairle...used with 
preposition  'a comarli' ..with intent, designedly....on the suggestion of"

 fo dáig --in order that, on behalf of

combad --   
 co   -- nasalizing conjunction used with subjunctive 'so that, in order 
  bad  --  past subjunctive 3rd singular conjunct of copula

furtacht, fortacht -- "act of aiding or succouring; help, succour"

dóib -- for them, to them 

ocon Tána na mbó a Cualngi -- at the cattle raid from Cualnge (literally 
at the raid of the cows from Cualnge) .   

>The variants are shorter --
>LL   Is iar sin luid Flidais co Fergus mac Róig. 
>EG   Is iarsin luid Flidais co Fergus mac Roig. 
>LFF  7  iarsin luid Flidos co Feargus mac Roith.