The main things you'll be dealing with for an Irish bass (or even alto) flute are finger stretch, weight, and size. With finger stretch on open tone holes probably the main limitation. There is a reason most modern Irish flute makers don't offer anything lower than a Bb flute, although I think Casey Burns makes one in A.
I'm 6'2" tall with proportionally large hands. The finger stretch is manageable on my six key Lehart low Bb flute, but I don't think I could handle the wider tone holes that would be required for a lower pitch like A or especially G.
The Lehart Bb is also huge for something played horizontally, at least compared to what we're used to with D flutes. I have to be careful waving it around that I don't knock something off the nearby table in my practice room. The Boehm "alto" flute in low G gets around this by folding the headjoint back around in a U-shape, and using keys to reach the appropriate tone hole length.
A low flute in traditional materials like Blackwood is also fairly heavy, because low pitch doesn't just require more length but also wider bore and increased barrel diameter I can manage holding my Bb flute horizontally for a few tunes, but it would be tiring for much longer. When I pick up my D flute after playing the Bb for a while, it feels like a piccolo!
So if you're going for a lower pitch than Bb in a bass flute while staying within the general confines of flute building (i.e. not a serpentine), then I think you might want a vertical design with a "swan neck' for embouchure. That is, if you can work out the finger stretch without keys.