It's great to hear that you are planning a trip to Ireland next year and are interested in joining in with the craic. I would say that local pubs are your best bet if you are looking for someplace to join in a trad music session.
Many local pubs on the island have informal trad music sessions where anyone is welcome to participate in. No doubt people will love to hear you play the mandolin.
For example, the Sunflower Pub
in Belfast has one evening a week of open mic (on Monday nights), which you might be interested in checking out. Another great spot in Belfast for trad music is Fibber Magees
Some of our other Community members may also be able to offer their suggestions on places in Northern Ireland where you can join in a trad music session.