First- just outta curiosity, are there any real stylistic differences between ITM and STM mandolin playing, outside of the differences between the genres themselves in general? Anything to do with ornamentation and all?
Second- picks. I know heavy and super-heavy picks are pretty common for bluegrass mandolin; what abotu Irish? I’ve heard they tend to use lighter gauges; I tried my friend’s light pick today- ew, but I could really see how that helps the triplets and such.
While I am an ITM mandolin player, I’m not an expert.
Mando is relatively new to ITM and STM. In fact, some would say it’s a little early to be talking about traditions and specific styles.
From what I’ve heard there are more similarities between ITM/STM mando styles than there are differences that I could put down to the different genres.
It’s made all the harder to be sure because styles vary so much even within ITM. John Sheahan and Barney McKenna of The Dubliners share a style while duetting, but catch them singly and John betrays a fiddler’s approach and Barney a banjo player’s bias in some things. Furthermore, neither of them plays like Andy Irvine who, in turn plays nowt like Eddie Furey or Anthony Warde etc etc etc.
As for the picks… ITM tenor banjo players tend to use light picks (.77 or lighter) as they allegedly help with rolls and triplets. The same is often true of ITM mando players.
That said, I personally have found that with practice I’m getting triplets down just as well with my bluegrass picks (modified 2mm Dunlop Gator Grips) as I did with my .77 Tortexes.
I must say, however, that there are times I go back to the thinner picks in order to get a sharper tone and less bass response for jigs and reels. The actual triplets etc come fine at both gauges.
The mandolin in ITM/STM doesn’t really have an ancient tradition to fall back on, so I don’t think the concept of stylistic tradition applies too well. At least not in the way one might discuss the harp for instance. Indeed, I’ve run to purists that don’t feel the mandolin belongs in a traditional session. That hasn’t stopped me from earning my living that way for the past 15 years though! 
I’ll agree with buddhu about pick gauges. I use thin picks on my Irish tenor banjo where the tripletts are easier on the long, low-tension strings, but thicker picks for mandolin where the strings are short and taught. The banjo is loud enough to get away with thin picks, too. I never use picks as heavy as are popular in Bluegrass mandolin. But picks are personal choices. Play what works best for you and gives the tone you’re after. They’re among the most affordable of all musical accessories, so you can try a bunch. Just try not to fall into the trap of always using what seems easiest today. The effort needed to get good with a different pick is a valuable learning experience and helps one progress as a player. Even if you don’t stay with a different pick you’ve learned something first hand, and that beats all the advice on the Internet put together.
excuse me for this “intervention” ( I’m Breton and my english is …what it is 
), but ? what do you mean by ITM and STM, please ??? Thanks for any answer. 
ITM = Irish Traditional Music
STM = Scottish Traditional Music
may thanks for the explanation Tim.
Regards
YVON 
And thanks you guys for all that information. Helpful 