I tend to try to sing different songs each time I'm in a particular setting (tho' inevitably, when some people know me, occasionally a particular one, or couple, would be requested), and since I'm aware that there are variants of many songs, bith with regard to words and melody, I'd tend to mention this at first, perhaps as a means of forestalling anyone taking offence, or maybe even joining in. In Ireland, I've always taken it as a mark of appreciation when people join in with the last line, or with a chorus, and am glad to hear this (and sometimes will reduce volume, accommodate tempo, engage the company with an encouraging glance, &c. when this happens).

With regard to some people having favourite songs for themselves, in most cases these are songs I wouldn't do myself - why sing a familiar one when you can introduce one that may be "new" to many people - but on one occasion I nearly got a really black mark. Thomas Davis' "The West's Asleep" (aka "The West's Awake") is a great song, and one of the ones I'd sing once well warmed-up and if it seemed appropriate to the company. Note the words "well warmed-up". That is, I would never start with it. On the occasion in question, one very old man sang it very well, and I later learnt that it was indeed one of his favourites, expected by everyone familar with the place. Thank heavens he did so early on in the evening!