I came to this discussion to ask this exact question. I’m hearing “ag” and “ar” the same in the VERY few questions that actually have audio (even the ones that indicate that there is audio).
Two things are against me here:
1). It’s a very new language to me, and I haven’t picked up the nuances of the way it’s spoken, AND
2). I don’t hear well, even when I’m listening to English (the only language I know). My wife tells me all the time that I need hearing aids, but that would mean admitting that I’m getting old (I’m pushing 50)...I’m just not ready to admit that just yet!
The "yow-ah" bit appears to be a dialect issue: I am learning Munster Irish and would definitely hear the "l" in local classes. The dictionary website teanglann.ie has a pronunciation tab where you can hear many words pronounced in the three different main dialects: the Duolingo speaker is using Connacht Irish as far as I can hear.
(DNTLS), …when one word ends in one of these letters and the following word begins with one there's usually no eclipse, so 'n' from "an" followed by 'l' from "leabhar" therefore no eclipse. You can sometimes guess where eclipses and lenition probably should happen just by speaking a sentence really fast like people in Ireland do with no gaps between words just a constant stream. That's why eclipses and lenition happen I suspect: it's a bit like taking a smoothing plane to a language so that words flow together easy and very fast.
It's got nothing to do with DNTLS - DNTLS is a lenition rule, and lenition only applies to b, c, d, f, g, m, p, s and t. You can't lenite l, n or r.
Eclipsis only applies to b, c, d, f, g, p and t. You can't eclipse l, m, n, r or s.
leabhar isn't eclipsed because you can't eclipse words that start with the letter l.