Good question, and actually, probably quite difficult to answer.
A lot of the tunes we know as reels, for example, originated as port-à-beul songs (“tunes from the mouth”). Port-à-beul is a lot like modern lilting, I guess, in which the singer uses fancy tonguing (basically) to articulate the notes. Port-à-beul was ported (to coin a phrase) over into instrumental traditions, particularly fiddling, and the instrumentalists developed rather complex gracing which imitates the articulations used in the port-à-beul. Hence rather complicated articulations, particularly in Scottish fiddle styles, on an instrument which has no trouble articulating notes without gracing.
On other instruments, it’s another story. Highland chanter fingering uses a system called half-closed, meaning that if you’re playing notes fingered with the top hand of the chanter, the bottom hand keeps its holes closed. That facilitates very complex gracenotes, all of which are named with their port-à-beul syllables. Maybe that’s not all that clear. What Irish flute players might know as an A roll, for example, is similar to the Highland chidere, which is a three-part gracing that sounds like CHI-de-re. ANYway, fiddles and pipes have no problem. Uilleann gracing isn’t as complicated as highland gracing, but there are definite similarities between the two traditions (with crans/cranns/bubbly notes existing in both, for example).
From listening to Irish flute players play their tunes, you’ll hear a very wide variation in the way tunes are graced. Chris Norman uses quite complicated gracing (he comes from a Scottish tradition); Molloy is also quite fond of very pipe-y sounding gracing. Other folks play different styles . . . one of the strengths of the Irish solo-ensemble model is that individual players can develop their own styles of gracing, unlike Highland pipe bands in which everyone has to play exactly the same gracenotes at the same pace for ensemble unity . . . man, I love to digress.
So I think there’s evidence that the gracing/articulation you’re talking about has a double origin. One: it existed in the mouth music which predated the use of the instruments. Two: it’s obligatory on Highland pipes, and it facilitates uilleannn playing, so those instruments (and fiddles) have helped to make it part of what we think of as the Irish/Scottish style of playing.
As for an originator, I think that’s a hard question. I’ve heard people more experienced than I muse that Molloy may be responsible for some of the popularity of uilleann-style gracing in Irish flute playing. I’ve heard some older recordings from the first half of the 20th century, and the few people I have heard definitely aren’t quite as fancy as Molloy is. I have by no means a representative sample, though, so who knows.
If you want to hear some VERY heavily-graced flute playing, listen to the piobaireachd on Chris Norman’s Caledonian Flute. Very nice. Really REALLY pipe-y.
Just my own thoughts.
Stuart