It doesn't really matter HOW you start, as long as you all agree on the point, and rehearse it a few times. My group does "South Australia", opening with about a 30 second bodrahn solo. This was actually about our hardest start, as we all just had to listen to the playing, and decide where a good place to start would be. Then, it was just a matter of remembering.

Another song starts with a very soft chord... helps us keep in tune, as well as notifies everyone when to go.

All this depends on rehearsal, though. A lot of the 'raggedness' in our group gets polished out that way.

M