I'll be traveling to Ireland with a friend who is an accomplished guitarist. During the week we are there he would love to join in with musicians in the pubs. First... is this even feasible, or does custom frown on him even asking? Second... if it's possible to do this, he would plan on picking up a good, reasonable guitar in Dublin when we land rather than travel overseas with one of his regular guitars. Any music shops or pawn shops in Dublin that you would recommend?
I have no idea, but how about having him check at the Acoustic Guitar Forum, guitar forum There are a lot of members from Ireland and the UK so somebody should have guidance for both questions.
No particular recommendation but in the city centre he could try Goodwins www.goodwinsmusic.ie or McCullough Pigott https://www.mcculloughpigott.com among others. You may need to check if anywhere buys 2nd hand instruments as I presume he won't be bringing it home with him. Otherwise a charity shop may accept it if they think it will sell.
Music aficionados may disagree but I wouldn't have considered the guitar as being common in a lot of Irish traditional music, particularly at smaller sessions.
I agree with FFS. At numerous trad sessions over many trips, I remember seeing a guitar but once. Two or three fiddles, a penny whistle or two and a bodhran are the customary trad instruments of choice.
I agree that I've not seen a guitar in the trad sessions I've seen. You may see a mandolin, or even a banjo occasionally. Usually it's just a fiddle (or two, or more), flute, and bodhran. But you may also see an Irish bagpipe, accordion or concertina.
Edited to add: Yes, you can ask if you can join in for a tune or two, but don't expect to spend the whole time if it's a regular group. Some pubs have trad nights where it's basically a jam session and anyone can join in. We encountered a couple of those and I was with some musicians who had their instruments with them and did join in.
Maybe things have changed, but when we were in Doolin many years ago, the sessions seemed to be mostly drop-in players, often Americans. We were slightly disappointed at the result.
When we were in Westport back in 2015 we went to a session at Matt Malloy's. A woman came in with a fiddle case, set it on the table and waited patiently. After a while one of the players spoke to her and eventually invited her to join in--and she was fabulous! She was able to keep up with the other musicians beautifully and ended up playing for the entire session. It turns out she was a violin teacher back in the US. Her dream had always been to travel to Ireland and play in trad sessions and she had already played in several others before this one. The look of sheer bliss on her face was priceless. Although that was the only time we saw an American join a session, clearly it can be done. We did see several guitars during the various sessions we attended--not as many as some other instruments but not uncommon either.