I know that there is no definitive answer to this but I’m interested in opinions anyway since I’m in a bit of a mando desert here so its hard to try things out.
A few folks say that oval hole mandos sound best for ITM…
personally I think its personal prefference… 
there are a lot of topics about this at http://www.mandolincafe.com. Check out their message boards there…they have a celtic music board.
Owell, if you don’t mind a train ride to Brighton, you might want to check out http://www.theacousticmusicco.co.uk/erol.html#1x0 (scroll down that page!).
I plan to visit that one when I have the opportunity!
Oval hole A style flat top in my opinion.
I like something with a good bit of sustain so my left-hand ornaments work. And loud so I don’t have to work so hard. Beyond that, I’m pickier about my setup than about the actual instrument. There’s a general preference for A-style round- and oval-hole mandos in the Irish music world, but I think maybe that’s just because they’re cheaper than F-5s.
From a complete outsiders’s perspective. . .
I like the sound of A-styles more in an intimate setting, but for melody an F cuts through better in a crowded session.
FWIW- mine is a Kentucky A style with F holes, and I like it just fine. F holes are cooler looking than ovals, IMHO.
The ideal ITM mando (to me) is a Gibson snakehead A from the Loar period (about 1922 to 1924). But paddlehead A’s will do quite nicely as well.
If you are on a budget, there is an oval hole Johnson A mando that sounds pretty darn good for under $100. But the quality varies a bit on these so try before you buy. Quite a bit of “cluck for your buck” as they say.
A friend has a Sobell (made in Northumberland UK). Ah, that’s a thing of beauty and power, all right.
Thanks for the replies, I now have a possible flatiron army/navy in the offing which might be just the job 
Congrats, Orwell. That should do it!