Okay, leaving aside the question of how representative the Irish Music Club was, I think the tunes speak for themselves, in being predominately in a range from the flute/whistle/pips D4 note and up from there. Of course, some fiddlers will want to explore the full range of their instrument, not just going below D4 but also using accidentals and modes difficult or impossible to play on pipes, non-keyed flutes and whistles. But the core repertoire of ITM doesn't reflect that.
For example, take a typical list like "
Dow's 50 (Actually 60) Essential Tunes." There are only two tunes in that list that drop below D4: "Farewell to Erin" and "King of the Faeries." Every other tune in the list is in the range above D4 (with a few like "Cliffs of Moher" that are in keys aimed at keeping them playable on whistles and flutes). If you did a database scan of tunes at thesession.org, I think you'd find a similar proportion of tunes reaching below D4. There just aren't that many.
To me, this suggests that either the vast majority of tunes were originally composed on the diatonic instruments that could only reach down to D4, or else they were composed by fiddlers who wanted their tunes playable on those instruments. And it's those instruments that made "D" the reference note for the common keys and modes used in the music. Not the fiddle, with all due respect to fiddlers. I live with a fiddler, so I have to be careful making this point.
