Cúmhaí wrote:
Interesting. As an aside, could you perhaps explain to me why sometimes ar lenites and sometimes it doesn't?
I like comparisons with German.

You could say phrases in German as "auf Provision", "auf Kaution", "auf See", etc., i.e. the preposition "auf" (= ar) used only with a noun without any article (and so usually without any trace of case marking)
In the same situations "ar" doesn't lenite in Irish:
ar cíos, ar muir (as FGB calls it: "In references of a general nature it does not normally affect initial letter of following noun")
But this is not an explanation, just another example in a different language.
The explanation is perhaps, that the noun is
"absolute" and generalized without lenition in Irish (and without a definite or indefinite article in German).
There is actually a three-part distinction in German and English:
sea, a sea, the sea.
But Irish lacks an indefinite article, so the distinction is normally only two-part:
muir, an mhuir.
But after the prepositions "ar" and "thar" a trace of a three-part distinction is visible:
ar muir = auf See, ar mhuir = auf einer See, ar an muir = auf der See.
Cúmhaí wrote:
So is adjective agreement the only place this rule is violated, or are there others?
One thing I commonly hear is the rule broken after an- meaning "very." I was always confident that was just wrong. Was I mistaken?
e.g. an-dheas?
It occurs, not often, but often enough even by native speakers.
In written Irish it is more often by Munster authors, perhaps because they actually say "ana-dheas" but write in standardized spelling "an-dheas".