Liz wrote:

> >"ar dorad Dîa do
> >Pâtraic 26) ar in 27) Crúaich in cethurda sa .i. gan 28) dîth ecnoi
> >29) a nÊrinn go deriud 30) in 31) domuin, et nâ trebsad echtraind ar
> >ois nó ar écin hÊrinn co brâth, et moir 32) dar Êrinn secht mblîadna
> >re 33) mbrâth 7 gorub 34) ê fén bas bretheom for Gôidelaib 35) i lló
> >brâtha." "Deo gratias", ol Bricîn. "Amen", ol int aingiul. 36)
> > 
>   DIL A 364.79 says “In Middle Irish ‘for’ and ‘íar’ are frequently
> spelled ‘ar’" and that looks like what is going on here.  

Yes, it must stand for "for" here.

>   ‘cethurda’ looks like “ substantive io, neuter four
> things, tetrad”.  I changed the article to ‘a’ because it is a neuter
> noun in the accusative case.  I thought ‘sa’ was the demonstrative
> pronoun.

Well, that would be correct if the text were OIr. But in MidIr. the 
neuter had largely vanished, so I would retain "in cethardae-so".

> The first thing:
> deriud looks like accusative of ‘deired’ (o stem neuter meaning
> ‘end’).  I saw the spelling ‘deriud’ in three of the dictionary
> examples so I kept it. 

"deriud" is rather the spelling for the dative, so we should perhaps 
better normalise to "deired".

> The second thing:
> ‘Et’ looks like Latin. ‘nâ’ looks like “conjunction...nor..used after a
> negative verb or ‘cen’ to to co-ordinate an additional member of a
> compound (or elliptical) sentence”.  

"et/ocus" "and" and "ná" "nor" are mutually exclusive, you couldn't 
combine "and" and "nor" in English, either. I rather think that "ná" 
is the conunction "that not".

> ‘Trebsad’ looks like a form of
> ‘trebaid’ (“cultivates, tills, ploughs...inhabits, dwells in”).  I see
> a few examples spelled with ‘trebs-‘ in DIL T 282.39 and 45, but I
> also see an f-future form, ‘trebfait’.  I didn’t know what to make of
> the ‘s’ in ‘trebsat’, so I ignored it (probably the wrong thing to
> do).  

That's difficult. "trebsad" looks like a preterite, but this would be 
out of place here in a prophecy about the future. I feel a bit uneasy 
about it, but i would do what you did, i.e. emend to "ná·trebfat".

> The fourth thing:
> Gorub’ looks like the conjunction ‘co’ and some form of the copula or

It's "co" "that" + "ro" + 3rd sg. subjunctive of the copula.

> I’m not sure I translated it correctly here.

I'd translate "and that he himself would be the judge..."

> The phrase ‘i lló brâtha’  looks like it should be ‘a llaa brátha’ (the
> article plus nominative singular of the neuter io stem ‘laa/lá/lae’
> and the genitive singular of ‘bráth’). 

No, "i lló" is actually correct. "i" is the preposition "in", and 
"ló" is the dative of "lae".