I have been listening to more Scottish music lately and I am wondering: how do you accompany Highland bagpipes? I’m thinking here of the Tannahill Weavers or Breabach. Do the flute players use a regular D flute and play one octave above? Is it another flute tuning altogether? A third option that I cannot think of?
Use a flute pitched in Eb.
And an amplifier
If you find a Scottish trad session to attend, the piper will most likely be playing one of the types that Highland pipers use to play along with other instruments in quieter indoor settings: the Smallpipes, Border pipes, or “Reelpipes.” These will be pitched in A or D and compatible with a D flute. I’ve played flute with pipers using these types and it works well, although I prefer the quieter Smallpipes. Border pipes can be pretty loud, although nowhere near as loud as the “outdoor” Highland pipes.
To the best of my knowledge - and I was there at the time - “Alba” were the first of the Scottish bands to incorporate an Eb simple-system flute with Highland bagpipes - Sean O’Rourke on flute, Alan MacLeod on bagpipes. The tracks from their one and only album are on “Youtube” . This is one of my favourite recordings of Scottish music ever :
https://youtu.be/9gEdoKiiPOU?si=jCoi5g-TO3gr_FRf
…and musicians earplugs.
Have you ever been up close or indoors with large pipes being played? 
A piper played large pipes at the indoor, low ceiling reception at my mother-in-laws wedding. It was akin to deploying neutron ordnance. 
Small pipes or Northumberland pipes is a different, more manageable, kettle of fish. 