The following is a very long-winded set of notes I wrote while nerding out before my workshop. I'm not sure how useful these are for folks, but I do encourage you to watch the videos.
Here is a link to the tune I taught - "The Ashplant"
Irish, Scottish, Cape Breton and Quebeçois fiddle styles (among others) all employ bowed triplets. At its simplest, a bowed triplet is an ornament comprising three short, percussive ("crunchy") bowstrokes on the same note. A triplet occupies one beat of a reel.
More complicated triplets coordinate left hand ("finger") ornaments with the same three percussive bowstrokes. You could play a run of three notes (e.g. "f-g-a") or a piping "cut"-like ornament (e.g. "B-d-B") with a triplet bowing. You can also adapt triplets to other types of tunes---hornpipes, jigs, etc. The technique differs but the principles remain.
I believe the majority of fiddlers tend to play triplets starting with a down bow ("Down-up-down"). I am aware of a handful who play "up-down-up". In his book Ó Am go hAm – From Time to Time, Tommy Peoples advises practicing both varieties to be ready with whichever is more appropriate. Let me know how it goes.
Some Video Examples
Sean Smyth (with Kevin Crawford)
Kevin Burke (with Micheal O'Domhnaill)
Tommy Peoples (with Paul Brady)
("Brenda Stubbert's" at 0:56)
How to do it
Set yourself up for success. Obviously, if you find that you are a "down-up-down" tripleteer like me, you have to line yourself up for a down-bow on the desired triplet spot. This is usually not as dicey as it sounds. Your subconscious bowing traffic cop will help you.
Then, relax your wrist and lighten your grip. To get a good percussive "crunch", you need some headroom in your bow. If you're already bearing down hard before the triplet, you won't be able to add much texture. Imagine a momentary lift right before the triplet starts, and curve your wrist accordingly.
The bow strokes themselves aren't bowstrokes in their own right. The three of them form a unit. Call it a reflex, an intentional shudder, maybe a purposeful spasm.
If you're trying them for the first time, your task is to weld the three strokes into this "unit". It's maddening. For a long time. But when you start to get it, you never want to stop playing them.
Then you have stylistic choices. As you see from the videos, some fiddlers have a fair amount of "note" in their triplets. Others are more "crunch". It's a matter of taste and to some extent the origin of the music.
Timing is another variable. You have a whole beat of music to work with. Some folks spread the three notes evenly throughout the beat. Others cluster them at one end or the other. Another matter of taste.
Lastly, you have to get out of this mess. As a rule, I slur the last bowstroke of the triplet into the next note of the tune—almost always. That's me.
Bear in mind, a different fiddler—someone who starts their triplets on an up bow especially—will have a radically different approach to these. Interrogate other fiddlers whenever you get the chance. You will hear some pretty strange ideas.
Lastly, like any ornament, playing triplets is only half the battle. Deciding where to put them is another subtle art. Your ears are your allies here. Listen to the playing of your favorite fiddlers. Don't try to analyze every note; rather, get a feel for how the the tunes flow together with ornaments and variations. Follow sheet music if it helps you practice, but put it in the woodstove if you find yourself depending on it.