Well, if I'm doing a gig at a pub or restaurant I'll usually get an
opportunity to interact with the patrons prior to going on, and, with
that in mind - I could start with almost anything. Since I don't write my own songs the coffeehouses won't book me; so, when I'm at a
a coffeehouse - I just don't start.

When doing festivals - Irish / Celtic - I like to start out with
something bouncy that will get toes tapping and heads bopping and such so I usually select one from the following list:
"Star Of The County Down," "Ballad Of St. Anne's Reel,"
"Real Old Mountain Dew," "Feels So Near," or "Roseville Fair."
Cheers.