I've been going under the assumption that "mh" is generally pronounced as "w" when broad and as "v" when slender.

However, the word "mhaith" comes to mind... I've heard it pronounced both ways. Is there a regional influence at work here as well?

The creeping loss of bi-labialisation some divergences are occuring

From wiki:

/w/ has two basic allophones: the labiovelar approximant [w] and the velarized voiced labiodental fricative [vˠ]. The distribution of these allophones varies from dialect to dialect. In Munster generally only [vˠ] is found (Sjoestedt 1931:28–29), and in Ulster generally only [w] (Quiggin 1906:74–76). In Connacht [w] is found word-initially before vowels (e.g. bhfuil [wɪlʲ] "is") and [vˠ] in other positions (e.g. naomh [n̪ˠiːvˠ] "holy", fómhar [ˈfˠuːvˠəɾˠ] "autumn", bhrostaigh [ˈvˠɾˠɔsˠt̪ˠə] "hurried": Finck 1899:64–67, de Bhaldraithe 1966:30–31).

There is a development where a 'broad v' and 'slender v' contrast; as far as I am aware it is like 'vw' in a bad Austrian accent, vs. an english v. Maybe that is what you heard.

I'm having deja vu here -I recall telling this before.

Anyway, just do a 'w' vs' 'v' distinction in all positions they are call for,a nd you'll be OK

PS: did you ever find the instructions on how to make a slender r useful?

Yes I did! The "Irish NOW!" audio computer program actually came to the rescue on that score. It's a sound that's terribly difficult to explain in words, you just have to hear it done.

It would be a cool project to add such functionality to this site, actually... what are the odds of that happening, I wonder? Would it be possible?

To hear most sounds of the Irish language, go there: http://www.phouka.com/gaelic/sounds/sounds.htm

To me, a velarised v is a nonsense in Irish. Broad vowels are velarised, except the bilabial ones (b, f, m, p, bh, mh, ph), which are followed by a short w: [bʷ, ɸʷ, mʷ, pʷ, w/βʷ, ɸʷ].

Note: [ɸ] is bilabial f, [β] is bilabial v.

In older speakers' Irish, mh is nasalised while bh is not.

"To me, a velarised v is a nonsense in Irish"

I would have thought the 'short w' would have sufficed to mark the distinction in broad bi-labials, as you point out.

Maybe the linguistics involved feel that the 'broad labials', if they were to be put in isolation, would have the tongue in a raised fashion, like a /u/ position, or perhaps they just got carried away with making the scheme nice and regular, so the labials had to have a specific 'velar' aspect.

"There is a development where a 'broad v' and 'slender v' contrast; as far as I am aware it is like 'vw' in a bad Austrian accent, vs. an english v. Maybe that is what you heard."

This was meant to convay the sort of pronounciation you hear taught to people where both broad and slender sounds are labio-dental (vw vs. v). I *suggested* it may have come back from creeping english influence, but I cannot say that as the dialect studies I have show bi-labial for Munster too, and make some mention as to younger spekers changing habits. Re-reading the passage it is not too clear that that was what I was trying to convay.

"In older speakers' Irish, mh is nasalized while bh is not."

Oddly enough, I've actually found that to be easier for me in my pronunciation of those - especially when slender.

O quiggin adds tho, that if slender mh is fully nasalised when slender it can pass into /m'/ so suimhneas =suimneas