I do The Unicorn if there are kids in the crowd, as well as Bill Staines' "A Place In the Choir," the latter on the strength that I heard the Clancys do it -- funny listening to "All God's craythurs got a place..." -- And fun to get the whole crowd doing hand motions on the chorus. (Of course, audience abuse is my specialty...)
One year the local Irish immigrants cornered me and asked me to go light on the rebellion songs, so I dug up lots of humorous songs and old (yechhh) love songs, only to end up catching hell from the Irish Americans -- where were all those good rebellion songs?
I'm not sure there's a Must Do, except the ones you like to play. But "Wearin' of the Green" always makes a great closer...