I’m sure it’s not an uncommon issue, so sorry if this is a repeat, but I’m having trouble finding info on this… I’m wondering how folks handle notes in tunes that fall well below the range of the flute. Oftentimes people at the sessions I attend will play tunes such as Tam Linn, Farewell to Ireland (the setting in which the A part descends to a low G), etc. My current way of dealing with them is to either sit the tunes out entirely, or at the most just play the parts which I can. I’m not really fond of that approach, however… Perhaps one can play some sort of “counterpart” or “harmony” a third, a fifth, even an octave above the notes in question (you can see how “up” I am on my theory)? Is there some systematic approach I can take to this without it “clashing” or otherwise sounding like crap?
Adapt those passages and play them an octave higher.. That way it’s sure not to clash with the melody. ITM as with many forms is largely monophonic. So just the melody and nothing in the way of harmonies. Harmonies can work and oft times if done with taste and discretion but I’ve heard quite a few that sound a bit naff. To be safe stick to the melody but up the octave for the low bits.
Fintan Vallely’s tutorial “Timber” recommends exactly that and gives some examples.
Really, the only question is whether you play the surrounding notes of the phrase up an octave too 
I’m a relative beginner to this, but I’m finding that the second octave D and E often blend better with the folded A, B, and C/C#, which seem to be the most common. If there is more than a a couple notes below D, I don’t bother - things start sounding too forced to me. At least at this early stage of my playing. I still don’t have my ear trained to hear exactly what good players are doing, so it’s a lot of trial and error.
It’s one of the things we have to live with as flute players, a limitation of the instrument. But listen to Matt Molloy playing “Farewell To Ireland” with the “Bothy Band”, or the commercial recordings of his, and also Seamus Tansey’s playing of “Paddy Ryan’s Dream” - hard to find fault with those, I’d say. Nearly all flute or whistle players would “jump” the octave, in those and similar tunes, in my experience.
Here’s a great example of folding a tune.
https://forums.chiffandfipple.com/t/in-spire-ing-folds/88613/1