I like to tune down to A for smallpipe backup, G for acoustic sessions, D for hardcore Irish and of course C for my own feature stuff. Allows for the same scales and some light chording. As far as staying in C and playing in G or A, that pretty much leaves the melody on just the A string, sort of like playing a dulcimer. D is doable, but you lose any open chording you might have if you just tuned up to D.
I can bar chord pretty well too, so in a quick key change, that is best, eh? That's where tuning lower helps, as it keeps you from getting too high up the fret board. Bari is inherently in that key, which is also fun to play, but my current experiment is using a sop in those lower keys. Light, plunky sound for sure!