See also posting: Irish Music in Japan - History
I am planning to research sessions in Asia generally - what they are like and if/how they are different from back home. Ideas and contacts would be welcome. Meantime here are notes so far on Japan, and there is another article on Bali.
Drop-ins: All the tobi-iri (fly-in) sessions I have been to are hosted by 'Irish' pubs. (Pubs labelled 'British' and 'English' generally have rock music - live or recorded - although in the 1980s-mid 90s, before there were Irish pubs in Kyoto, the session was at the 'Pig & Whistle - British Style Pub'.) Many of the pubs will allow an impromptu session - they don't have entertainment licensing issues here - and keep house instruments you can borrow eg a house guitar and whistle, a (playable) bodhran on the wall. Often an open session will follow a raibu (live performance) by a well known player from abroad.
Gig sessions: Some so-called sessions, in Kyoto at least, are more like a gig - with PA and a stage area and you are expected to address the audience. It may be useful therefore to know what English language songs and melodies are familiar to Japanese people. More about that here.
Audience participation: Although Japanese people have a reputation for seeming shy or reticent (standing out is bad bad bad!), that doesn't apply to singing and they love to join in a chorus. As it's a foreign language, songs with simple refrains work best. They will also clap along a lot - which can be surprising but it's all in generous spirit - you might get used to it. On those two counts the song that goes down best so far is The Rattlin' Bog - even if they have no English at all, people laugh as they realise the verses are getting longer and longer.
Japanese players: In an open session by Japanese people the repertoire is 99.9% tunes, as opposed to songs. So far I have only ever heard songs sung in English by Japanese young women who had either lived in Ireland or are working in a band. In Kyoto the players are mostly flute and fiddle players, and in their twenties and thirties. If they played other music before this it usually classical on these same instruments, though some come from shakuhachi (Japanese flute). I have met box and concertina players in Kobe and Tokyo. I haven't come across any players yet who grew up with Irish music at home, school or Ceoltas. They learn from recordings, workshops from visiting players and some make trips to Ireland, or classes at Field in Kyoto given by the best Japanese players. There are a very few guys, now in their forties and fifties, who have been playing Irish music for 20 years or so and I have also met other older (male) players playing guitar, bouzouki or mandolin who have a background in bluegrass or C&W, which is still quite popular here among their generation.
Etiquette: Music-wise, pretty much the same as in UK, and as an overseas player you will be welcome and unlikely to tread on anyone's toes. The pace is not so fast as London - music or drinks-wise. Drink buying is different too - if you buy people a drink, don't expect one back - they don't do rounds here. And if it is supposed to finish at 11pm, it will.
Sessions: Links to session information
Irish Pubs/Sessions Kansai
Irish Sessions Tokyo
Session video from the new Cock O'The Walk, Tokyo
Workshops: Most venues will welcome workshops. To offer or attend workshops in Kyoto contact Field (which also sells CDs and tune-books) - they have classes and practice/recording studios. For other areas contact The Warrior Celt (Tokyo), Kells (Mito), Tokyo Fiddle Club, or any of the other venues/organisations (see 'Japan Venues' Links list).