"His reaction was to say that it was my fault I didn't sing in time."

I speak as one new to the club but also a bodhran player (of many years experience) and an unaccompanied singer. Generally I believe that it is not a good thing to mouth off at another musician - I am not saying that I haven't done it! Certainly on many occasions I have had to suggest to other precussion players that a particular session is not big enough for more than one percussive instrument at a time and suggest that turns be taken.

As for accompanying songs - I would never do it unless asked to or unless I knew the singer very well. And as for unaccompanied songs. It is usual and, I think, desireable for a singer to go off beat in a slow ballad accentuating phrases and lines. Think also of a slow air played by a solo instrument; quite often a phrase is stretched hence going off beat and beneficial to the tune.

While maybe undesireable, I believe that you were well within the limits of session etiquette to stop mid tune and have words. And maybe now this subject will be brought up at sessions all over and talked about freely in the open so ALL musicians can take on board that there is such a thing as session etiquette.