I've been working on learning mandolin on-and-off for a few years. Mostly off. I've just picked it up again. My background includes Highland Bagpipes (i.e., Scottish, not ITM). My wife, a good violinist, has been working on fiddle, on-and-off, for decades. Since she retired, fiddle is getting more emphasis.

I play whistle and Scottish Smallpipes, and on some tunes she's written parts to accompany me. It's cool. We're /not/ trying to do Irish session playing. I feel like I need to repeat that because some people will probably feel compelled to learn me on authentic Irish session playing, even though I have no interest

What I do want to do is learn how to accompany fiddle tunes on mandolin.

I'm also learning to play Led Zep's Battle of Evermore. Funny thing: on the internet you can find any number of people who have figured out pretty detailed transcriptions. They've even transcribed and learned to play the tape-delay effect.

Anyway, if anyone has any good resources or advice for learning to accompany Irish and Scottish tunes on Mandolin (i.e., devising arrangements, not session playing), please share.

After years of playing the flute, attempting the fiddle and getting decent at a certain number of concertina tunes I too found the itch to take my old mandolin out of the closet. I'm playing tunes rather than accompaniment, being completely intimidated by the complexity of hearing and arranging chords. That seems funny to me since I can play melody lines by ear all day... But I would think if you are trying to play accompaniment you might dig out some of the early Andy Irvine performances on YouTube, though their stuff comes from the Irish Tradition. He and Donal Lunny seemed to go from bouzouki to mandolin occasionally back in the olden days. Freeland Barbour played bouzouki with the Scottish band Silly Wizard back in the day as well. Perhaps studying some of his recording might help you.

I play ITM and practice largely to play with sessions, theoretically, but in practice I mostly play at home, by myself. I take online lessons taught by Marla Fibish on Peghead Nation. When you pay for the lessons you have access to all of them - videos, tabs, etc.. She starts off covering basically strumming for reels, then moves to jigs, but she also starts adding tips and tricks for accompaniment as she goes, and has a series of lessons that are all about accompaniment. I've found articles about accompaniment before, but Fibish's lessons are the first that have helped me improve.

While I’ve never heard of anyone doing this on mandolin (but it’s certainly a viable idea since you’re not planning on doing this in a session), something that might work is to listen to how some bouzouki players do counter-melody. You might need to retune to GDAD or ADAD if you go that route.
Zan McLeod has a DVD that goes into this some, if my memory serves correctly (it’s been a few years since I last watched the DVD).

I have looked at the Peghead Nation teacher's lessons and she seems good. I also checked out the short course on the Online Academy of Irish Music. The Peghead Nation site gives you a month for free, and I think the Online Academy of Irish Music has a trial deal as well. The tutor on OAIM was very specific on picking patterns, which seemed similar to those of Irish Banjo. I am a hack at Mandolin and didn't put any time into it, focusing on other instruments, but they are a decent resource out there. There was a fellow on YouTube who was giving free lessons who wasn't half bad as well.

I don't know if this counts as ITM - I grew up believing it to be an Irish folk song, but my favorite rendition is a Scottish musician's (Bert Jansch). I play mandolin and sometimes try to transfer something from guitar if it really grabs my heart. I started as a classical viola player so there are things in folk and pop that just don't occur to to people like me - and Bert is brilliant, so my ears absolutely could not decipher what he was doing. Then I found this particular tutorial and when he points out the "bend", it clicked. I had tried F and F# and they both sounded wrong - it took a bend - so... the string fret addresses will have to be translated to mandolin, but otherwise this tutorial helped me learn Wild Mountain Thyme, the way Bert plays it, on mandolin. I should say "Bert's version", not "the way.." because I could practice every minute from now until I die and I won't do anything the way Bert did...

https://www.youtube.com/watch?v=TuahnXb ... dex=5&t=0s

I used to play some whistle and have taken it up again, but I’ve also embarked on the mandolin. I bought a Waterloo round-hole mandolin and am playing fiddle tunes, or essentially the same pieces I play on whistle. Haven’t played chords too much, a couple of simple songs, but I’m finding TIM to be really fun on the mandolin. Just really common stuff so far: Swallowtail Jig, Sgt. Early’s Dream, Rights of Man, but music has become essential during the quarantine and the mandolin makes me smile.