Neil McLeod wrote:

OK, I'm going to do this with a couple of glasses of Markowitsch  
GrŁner Veltliner on board.  Wish me luck!  :-)

> LL   Mani thrŰethad a ara

if not * he were to have subdued (past subjunctive of "traethaid") *  
his * charioteer

>     ro-festais a mŰrgala;

they would find out / might have known (past subjunctive & conditional  
of "ro∑finnadar") * his * great X(??)

>     ropad re Temraig tÓr

(it) would be (3rd sg. conditional of copula) * before * of Tara * land

>     resfed carpat a d‚ rÓth.

would run * chariot * its * two * running(s)

I'm not sure what to make of the "its two runnings", but then again,  
I'm not sure we need to be bound by the supplied length marks here.   
The MS has "adarith", which might be divided as "ada rith", with  
"ada(e)" as the adjective meaning "fitting, suitable", and "rith" the  
noun "running", giving us the figura etymologica "resfad rith" =  
"would run a running".

The sense I make out of this in my current frame of mind is that our  
man had to whack his charioteer, otherwise "they" would have found out  
that his excursion did not emcompass Tara.  Roughly:

Had he not subdued (i.e. "killed") his charioteer,
they would have found out about his great [blunder];
(otherwise) it would have been before the land of Tara
(the) chariot would have made its fitting run.

OK, Neil, your court.