Dave - As far as I an concerned there is nothing more boring than half a room of people playing EXACTLY the same notes. They don't do it in orchestras where REAL musicians live.
Or to put it another way, different instruments can play particular patterns of notes easily whereas on a different instrument the same combination can be a very awkward fingering. Tin whistles add trills and rolls which are almost impossible on a fiddle, a melodeon or 5-string banjo.
If you know what to look for, and analyse something such as O'Neils Music of Ireland it soon becomes obvious that some tunes were collected from a fiddle player, and others from tin whistle. You can tell the difference from the style of ornamentation.
The outcome of this is that any tune consists of a basic melody line, which people have then added frills that suit their instrument, and perhaps a "regional style" of playing them.
What happens in sessions is often taken back to a simpler version, then the "flash players" either speed it up, or add their own decorations.
As long as you play notes where they appear in the "basic" version, you will fit in. If you play chords or notes that appear in the accompanying chords, you will likewise fit in. If you know how to do it, you can play bass runs, or countermelodies, and you will make the overall sound of the tune better by filling spaces in the overall sound.
Personally, I often like to play rhythmical stuff, that make the tunes more interesting to me.

All in all, it's just a session.
Nobody's listening...
AND Unless you are playing Highland Pipes NOBODY CAN HEAR YOU...

Quack!
Geoff the Duck.