One of the session I attend sometimes has up to three guitarists attending (usually two). Aside from them taking turns playing, are there any ideas for how best to deal with this situation? And a related question, what’s a logical second ITM-friendly instrument for a guitarist?
Whistle! They’ll learn a lot more than if they took up another accompaniment instrument, I’d say.
Is the problem that they’re all playing back-up and can’t handle melody?
mando
mountain dulcimer 
Banjo. A tenor for Irish music or a 5-string for American.
Perhaps suggest a rule. No more than one guitarist at a time plays backup. Anybody can join in on melody.
They might actually take the hint and go away and learn their instrument.
Yep.
I’ll go with Mandolin too. Maybe octave mandolin (I’m thinking of building a kit version of one of those)
There ya go.
I don’t think it’s much harder to play tunes on guitar than on mandolin or tenor banjo, except that it can be a bit of a challenge to play the very fastest tunes up to speed. Light-gauge strings can help with that, since volume is not that much of a concern in ITM, where you probably want to blend more than to stand out.
It’s also cheaper to learn an instrument that you already own.
They’re all good guitar players and know how to accompany Irish music…the only problem is quantity.
There is a view that I incline towards that you shouldn’t even try to accompany a tune you can’t play the melody to or at least lilt. But any competent guitarist who can lilt a melody can learn to play it.
My earlier post was a bit facetious but I meant the bit about playing melody. You’d be surprised how many people playing ‘celtic guitar’ don’t seem to see the need to know the tunes. You can’t get away with it in any other style of music.
Yep. I even had one fellow have the gall to whine about players changing keys during sets. Your usual gizmoist’ll get a deer-in-the-headlights look when confronted with tunes like The Princess Royal, The Blackbird, or even An Rogaire Dubh, which is a basic jig, fercryinoutloud. Experiences like that usually don’t seem to prompt them to do anything about getting to know tunes in a serious way, though, which baffles me. Just because you can play chords doesn’t qualify you to do so.
End rant. For now.
Well, I would not be surprised!!! But even if the chords are right, there is something to be said for playing them in the background and not whanging away loudly while grooving on one’s own in the Celtic magic or whatever. I cannot hear other instruments well on CD’s and in performances. The chords jangle my nerves, they distract me, they don’t seem to have much to do with the other music. I guess the melody players want it that way, presumably they are giving instructions to the backup players, but why do they want the chords so loud??? Give me a recorder or a bodhran any day.
Bass, as long as you don’t try to get too fancy (or too loud) with it.
Aw, geez, it’s not like they’re gonna change from a sharp key to a flat key or something. I thought every guitarist in the world knew how to play one-up (G) or two-up (D), which is practically all session tunes, and if they change keys, it’s usually from one to the other. Maybe sooner or later he’ll figure out for himself that the session leader is barking to signal a key change, not because he has Tourette’s. 
At first I thought you had written An Rogaine Dubh, which is Irish for what? Black Hair Restorer?
True.
Then presumably they’re pretty familiar with the sound of all the tunes that are commonly played at your sessions. That should make learning to pick the melodies even easier.
The problem I would have if I ever wanted to play Irish music with others is that I only know about 30-40 melodies, and most of those are slow airs or songs.
The GREAT thing about a mando is that, when learning tunes via the noodle method (WHich is the best way IMO), one can noodle quietly.
I also happen to like chordal backing on a mando, but maybe I;m weird that way.
A bouzouki might be a good choice as well, because the tuning and looooong scale length combine to discourage mindless full-chord strumming, and the jangle factor makes it sound great both in trad and on those Byrds tunes you may play when you get home from the session.
I briefly owned a Rick 660/12 - solid, but other than that the same guitar used on those albums. Awesome guitar. Trad tunes sounded great on it. I had to sell it to fund a flute 
I hate it when backup guitarists (or any other backup instruments) are playing the wrong chords and the wrong rhythms for the tunes. With the fiddles there is a simply solution for lowering the volume, namely a mute, which is placed on the bridge to soften the tone. Is there such a thing as a guitar mute? Why not invent one? Another suggestion is to make up a session tune book, so that the accompanying guitarists can learn the correct chords when they are practicing.