We were in Ireland this May, and we had good trad sessions for 17 out of 20 nights. I planned our entire trip around music pubs using the website https://thesession.org/sessions/search?day=&q=sligo and by visiting individual Facebook accounts for the bars. The three strikeouts were in Portrush and Dunfanaghy and the Facebook info was a fib. Depending on the month, music is not necessarily nightly (May, June, Sep).
I juggled our itinerary to land in towns on the nights I could confirm music. Doing it 6-9 months in advance can be challenging :-) Luckily, many pubs have long-standing music nights, often year round.
Procedurally, I look for towns I want to visit, then find the most likely music pubs, then search for lodging within walking distance of that/those pubs. You'll have a more complicated procedure since you'll be public transit bound.
Our second favorite thing is Neolithic and historical stuff, which is often not on a bus route, which is why we always rent a car.