I read (like every Sabbath ) An Bíobla Naofa.
There it came to pass that I found this (Gníomhartha / Acts 20:35)


In English (KJB):

mar a ndúirt sé vs. how he said
Why is there eclipsis?

I’d expect mar a dúirt sé (and other Bible versions actually have mar adubhairt sé)
I’d translate mar a ndúirt sé rather as "where he said". Where? In his words?

Peadar Ua Laoghaire’s version:

mar go ndubhairt sé = because/for he said

That makes more sense because of:
Latin: quoniam ipse dixit = since he himself said
Greek original: ὅτι αὐτὸς εἶπεν = since he said

And some other English versions have "because" or "for"

This is common in Ua Laoghaire's writings, even if in that particular passage he doesn't use it (not forgetting that Gerald O'Nolan edited the Acts of the Apostles and may have imposed his own grammar).

In Acts 28 near the end (no verse numbers in my file), there is this:
"Is maith adubhairt an Spioraid Naomh é le n-ár n-aithreachaibh tré bhéal an fháidh Isáias, Mar a ndeir sé: Imthigh ag triall ar an bpobul san agus abair leo:...."
And "Ionus go gcómhlíonfaí an nídh adubhradh tré Isáias fáidh mar a ndeir sé" and numerous other examples.

Ua Loaghaire does have "mar a dubhairt" in many passages, as well as "mar a ndubhairt" in others. I think this may reflect contamination from the use of "mar a +Eclipsis" to mean "where". And these examples above may be parsed as "where". Is maith adubhairt an Spioraid é, mar a ndeir sé - "well did the spirit say it [in that passage] where he says...."

In your example, "cuimhneamh ar ráiteas úd an Tiarna mar a ndúirt sé gur ", "ráiteas úd... mar a ndúirt". The saying... where he said.

So it is fundamentally an indirect relative.

It is funny that three versions of mar occur in different translations:

• mar a dúirt sé, how he said
• mar a ndúirt sé, where he said
• mar go ndúirt sé, because he said

I don't think it's just that "mar" can mean "where", but also that there is cross influence from the use of "mar" to mean where, so "mar a nduairt sé" means "as he said", but under crossinfluence from the where use. This is mainly only seen with the verb abair as far as I can tell.

Interesting
There’s another form with the meaning "how he said" – at least acc. to GGBC – without mar but with a comprehensive relative pronoun, forainm coibhneasta réamhtheachtach, meaning otherwise "all that".
a ndúirt sé = mar a dúirt sé

Could this play a role here?
mar a ndúirt sé = lit. "as that-what said he"

Yes it could. It's be interesting to see what An Lon Dubh has to say on this.