It seems you're talking about bad/clueless instrumentalists rather than any instrumentalist.

Since your wife already asked him to stop and it resulted in an aggressive reaction from HIS wife, you've got a choice of a confrontational response, ignoring them and going on anyway, or stopping. You can always ask the session leader for help later on, but in the middle of a song, you have to make the decision, and what you decide depends upon your personality and what you think the results of a particular choice would be.

1. Confrontation will cause a reaction, and probably escalate piss-offedness in somebody who already seems determined to be obtuse.

2. Ignoring them is probably what an exceptionally calm and rational person does. (Whether 'exceptionally calm and rational' is appropriate to your circumstance is something you'll have to decide.) It sounds like the man and his wife are oblivious to the fact they should be embarrassed. Help the process along by varying the tempo a bit. They'll likely not notice, or have a clue about how bad they sound, but everyone else will, and may later give them a bit of feedback.

3. Stopping in the middle of a song may cause even more embarrassment to the non-rhythm players, but again, they may not realise it. It would have about the same effects as #2, but may come off as a tad passive-aggressive. You may end up being less irritated than if you kept singing, but you may be more irritated.

Another option would be to find a competent rhythm player who can manage just a bit more volume than the PITAs, and ask him/her ahead of time to jump in if needed. You can focus on him, and the bodhran and shaky-thing become background noise. (Hopefully)