Liz wrote:

> >a)  "Forremdeth in coibden hisiu a m-blednai cusin
> >     samain aitherrach.  Do-tiasait aidqi shamnai
> >     aitherruch, ar it aursliuctiu sido hErinn dogres
> >     him s*amuin."
> >
> >a1) "Foremteachair ... hisin ... atharruch, arit
> >     fosgoilti sidha Erenn dogres."
>    "For·émid techt in coibden hí-síu i mblíadnai cossin samain
> aithirriuch.  Do·tíasat aidchi s*amnae aithirriuch, air-it airslaicthe
> síde hÉrenn do grés imm s*amain."
>   I thought 'Forremdeth/Foremteachair' was the 3rd singular present
> indicative of the verb 'for·émid' (cannot, is not able).  According to
> DIL F 220.23, this is used "with verbal noun or equivalent as object".

I think we can find a solution which stays closer to MS a). "Fo(r)·éimdeth" is formally the 3rd. sg. past passive (DIL F 
220.75), maybe here the passivisation of an impersonal construction: "fo[r]·éimdeth in choibden" "the warrior could not 
(come)". The strange thing is that "fo(r)·éimid", as Liz said, usually takes a verbal noun as object (or as subject in a 
passive construction). But there is no trace of one here (I am not sure what "foremteachair" in a1) is, but I doubt that it 
contains a verbal noun). Instead, the subject of the passive verbal form apparently is "coibden", which normally means 
"warrior band". But maybe we can solve that, too. DIL C 288.58 says that "coibden" may sometimes be used in the 
meaning "marauding expedition", which semantically equals a verbal noun. If that's what is the case, "fo[r]·éimdeth in 
choibden" would mean straightforwardly "the marauding expedition was prevented (this year)".

>  I put in the verbal noun of 'téit', which is 'techt,' because source
> (a1) suggests it.   

Reading "coibden" as quasi verbal noun makes that unnecessary.

> I thought 'a m-blednai' was the Old Irish equivalent of 'i mbliana'
> (this year) in Modern Irish.  So I used one of the dative singular
> forms from Thurneysen GOI p. 185 and the preposition 'i' here.

That's another instance of writing "a" for "i", about which we talked just last time.

> I used the spelling of 'aithirriuch' that we used before in
> paragraph 3, line 3.

Yes, that's correct.

>  I thought 'Do-tiasait' was the 3rd plural present subjunctive of
> 'do·tét' (comes).  Does it always have to be translated as a wish?

In principal clauses (as in our case) the present subjunctive "is used for commands where immediate compliance is 
not contemplated" (GOI 329).

> Using 'may they come' or 'let them come' doesn't make sense if 'they'
> refers to the army of the fairy mound.

No, it refers to the real world army of Crúachain, doesn't it?

> I used 'samnae' because we used it in paragraph 1, line 1.  But I
> don't know why there is lenition here.  Do dative nouns of time 
> always do this to the following word?

The dative singular of all declensional classes in OIr. causes lenition.

>    I thought 'it' was the 3rd plural present indicative of the copula.
> According to Thurneysen p. 484 this is preceded by 'air' which he
> calls a "prepositional prefix."  I copied his spelling, including the
> hyphen.

Well, if Thurneysen does it that way, we can follow him. 

>   I thought that  'aursliuctiu/fosgoilti' was the past participle of
> 'ar·oslaici' (open) (DIL A 407.76) which has an older form 'ar·os-
> ailci' mentioned in Thurneysen p.525.  'Fosgoilti' looks like a newer
> form, closer to modern Irish 'oscailte' (opened/open).  'Airslaicthe'
> was given as the participle in DIL A 408.11.

Yes, I think you chose the better (= older) form. But note that the past participle can be inflected as a normal yo-stem. 
Since the subject (síde) and the verb (it) are in the plural, so must be the predicate "airslaicthi".

> I wrote 'síde' as the nominative plural of 'síd' because it looked to
> me like we have been using the s-stem forms for the word so far.

Yes, it is an s-stem.

> 'Érenn' is given as the spelling of the genitive for 'Ériu' in
> Strachan p. 15.  I kept the 'h' before it, but I'm not sure why it's
> there.

A very good question! I don't want to go into any etymological speculations, but it can be observed, that the name of 
"Ireland" is regularly spelt with "h" in MSS like LL or LU, even where one would think the "h" to be completely out of 
place. I even came across the spelling "i nhÉrinn" once.

> I did not keep the 'h' before 'imm' because I didn't see it earlier
> in our text.

Yes, that's fine.