Ok, I have said that the triplet is not the be-all and end all of Irish tenor banjo playing. John Carty says so. Gerry O'Connor says so. "But yeah", you reply, "They can afford to say that, I can't do a triplet!"
So the triplet, in spite of everyone who is anyone in the business telling you that it is "not that important", really is to most aspiring banjo players. So I guess I will try to help a little.
First, we define.
If you are a music major, you know that a triplet is three notes in the space of two. We see them in hornpipes all the time. But this is not what we mean by a triplet. What I am referring to is an rhythmic device or ornamentation that emphasizes the music. On the banjo it has a rapid Brrrpp sound that may not even be musical as it can be played on muted strings or while the left hand is in transition. It has nothing to do with the musical triplet. In fact, a number of musicians, Tommy Peoples and John Carty included, prefer to call them "trebles."
Ornamentations are integral to Irish traditional music. You almost never see them written into notation of Irish music (you will see a little ~ sign over a note that should be ornamented or an occasional grace note thrown in) because the ornamentation is a moving feature. Listening to master class players you will notice that each time they repeat a tune, it is different both in emphasis and ornamentation. If you play the same way each time, it is boring, and that reflects on your personality.
One problem that the tenor banjo has is that it is very limited when it comes to ornamentation. The triplet/treble is by far the most commonly used ornament because the banjo can do it so well. A banjo can produce something like a cut and possibly a roll but that is about it. So everyone has to learn to play a triplet/treble eventually.
Right Hand Position
If you are going to do trebles well, that is quickly and effortlessly, you have to have a good right hand first. This means you wil have to have an efficient and consistent stroke that produces good tone and volume. Before you can play a triplet, you have to have good right hand technique (see left.) In addition, you have to be relaxed and that is not easy sometimes, especially when you are trying to look good and play faster than you normally do.
When To Play a Triplet
Too many triplets sounds like, well, too many triplets. The triplet is not a musical entity, it is a rhythmic tool used to drive the tune and to make it more interesting. You can play Irish music perfectly well without the triplet and it will sound good. In fact, until you have mastered the triplet, that is the way you should play the music otherwise you will sound like you can't play the triplet.
A (very) broad rule of thumb is that wherever a quarter note appears, you can play a triplet. The problem is that much of the time the quarter note may sound better in the tune. And extended treble can be played on a dotted quarter note in jigs.
It is hard to play trebles when a reel is played at Warp 9 because there is just not enough time for a normal human being to put in the ornament. In that case, don't.
After a while, you will play fewer trebles in more tasteful places. Once you develop your own style, you will know what to do.
How Hard Is It?
Until you can do one, and there will be an epiphany at some point, it is very hard. The trick is to learn to do the techniques slowly and then add speed, but not too much. I took about three months of practice to get it so I could insert a triplet in our session without losing the beat or track of the tune. Once you are able to play with some confidence, it is a lot easier unless you think about putting a triplet in and then it becomes hard again.
The real trick is to listen a lot. Get as many CDs of Irish music as you can afford and immerse yourself in the music. Sing along and after a while you will be putting in triplets and other ornaments naturally. If I could only buy two artists, it would be Angelina Carberry who is both tasteful and subtle which means she uses triplets just right. The other would be Chris Smith whose Coyote Banjo Trio album is perfect for learning tunes (there are 44 in the CD) and tasteful triplets.
Triplets should just be there. They are not meant to be hard, and when you look at the sources on the left, you will see that the techniques are simple, they just take practice and most of all the ability to hear them in the music.
Added information:
I have received a few questions regarding the differences between the wrist triplet and the McTwist. There is an obvious aural difference which is due to the changing angles of the McTwist. In order to do the McTwist you have to bend your thumb and finger which automatically changes the angle of the pick to the string. With the wrist triplet, you keep the same angle and preserve the same tone.
If you use both techniques, you can use this difference in tone to make a statement. The placement of triplets remains the same, but you double the listening pleasure by employing both methods.
Here is a video of me playing the last part of "Lark in the Morning" using the McTwist triplet.
Here is another youtube example of the McTwist