I live in Ireland. If you were to speak during a song, the whole place would make you shut up! Unless self accompanied, or specifically requested, solo singing is the norm.

At a session, you politely ask to sit in. I play guitar, sing, and play anglo concertina. I wouldn't join in on the concertina if I didn't know the tune, so why would I accompany on guitar if I didn't? I'd ruin it just as completely.

If I encounter a complete fool, I pick a special song, and tell them "B flat minor" etc. Then launch in. If there's another guitar, or a piano, I don't play chords, just the melody, or countermelody. Nothing worse than chord soup!