Just found this thread. (and though I love Irish fiddle music, the discussion seems to have gotten a bit detoured on the Donegal style details).

I agree bowing is very important (along with playing in tune!)
There are some good reasons to use a classical hold - it works pretty well. Some people can develop a fairly serious injury to the thumb from the "under the frog" hold (not everyone) and there are some things that are just more difficult to do with the "grab it up the stick" hold. That said, plenty of people do both of those things very capably- but I'd still suggest starting with the classical hold.

It's good to be able to use the whole bow- even dance fiddlers who play fast and furious with short bows will want to play a waltz.

Scales with all different kinds of bowing patterns can be good ground work. You learn to hear if you're in tune and you can practice any kind of tune- waltz, polka, jig, reel etc depending how you vary it. Since you don't need to think about a tune, you can really listen to the sound you're making.

Personally, for fast fiddle tunes, reels, using short bows above the mid point works for me.

One of the most important aspects of bowing is how each style of fiddling has characteristic styles- whether separate bows, double-stops, where the slurs are, speeding up the bow for accents etc. That is the part of fiddling that comes from listening and watching- every style is a separate language or dialect. So it's hard to offer general guidelines. Listen, listen, listen... and watch/ ask other fiddlers.

Hope that's helpful....