In music sessions especially English Country Dance band style ones, a bit of percussion is usually welcome. For some of the slower tunes especially things like O'Carolyn waltzes pecussion can be a bit of a problem unless the players are good.

In song sessions any extra accompaniment should normally only be with the permission of the singer. Obviously certain songs (ie Wild Rover) are usually suitable for everyone to jam in on, but keep your eys on the singer(s) - DON'T TRY TO CHANGE THEIR TEMPO/RHYTHM and if they're looking daggers at you - DESIST. I once had a bodhran "player" insist on thumping away when I was singing "Grey Funnel Line" - afterwards she told me that she'd only just bought it and wanted to practice as much as possible. I suggested a suitable place - which was eminently suitable as it had a supply of tissues for the flood of tears which my advice produced !