On 17 Jan 2012 at 21:12, Dennis King wrote:

> > LU     ar do  irgartigud a menman & do breith táircthe cruid o  
> > Ailill & o
> > Meidb.  luidi síar co Cruachain
> 
> Since we had "iar tíchtain dó" in the previous sentence, I'm going to  
> assume we have a continuation of the same structure in these lines.
> 
> ar do = iar dó = after for him
> irgartigud a menman = entertaining of his mind
> & do breith = & (iar) dó breith = and after for him taking
> táircthe cruid o Ailill & o Meidb = of provision of cattle from A and  
> from M  ("táirciud" and "crod" in the nominative)
> luidi = luide = that he went (relative of "luid", preterite of "téit")
> síar = westward / back
> co Cruachain = to C

I think the analysis is different: 

ar = because
do irgartigud, do breith = these are two "do"-infinitives, consisting 
of the preposition "do" + verbal noun

"because it is to entertain his/their mind and to bring a provision 
of cattle from Ailill and Medb that he went back to Crúachu"

David