Irish session musi, f'rinstance, eschews harmony an counterpoint---which makes it agonizingly dull to my ears.

A few things here. First, sessions are a relatively new tradition. The majority of Irish tunes predate the modern penchant for playing them in sessions. They didn't start their lives as "session music". Second, playing tunes in a session is not primarily, or even at all, about providing a satisfying listening experience for an audience (though it's a great bonus if it does, and, in my view, it often does, but that's a personal opinion). Third, you might have experienced some pretty rigid sessions, but I don't think it's generally true to say that sessions eschew harmony. You won't find too many sessions that are melody instruments only playing in unison. As soon as you introduce guitars, mandolins and other stringy jobs, you have harmony (and there are, of course, instruments that can do double-stopping). What you don't get much of in sessions is arrangements (apart from unconscious things such as dropping in and out when you know or don't know the tune). That's the difference 'twixt a session and a band.

I'd argue that you don't actually need to embellish Irish tunes with too much harmony, if at all. A bit like Bach's unaccompanied cello suites, which have many passages without even double-stopping, harmony is "suggested" in the mind's ear when you hear the tunes well played. Not that I'm saying that Irish tunes as as good as that, but the argument still applies.