Guitarist. That is the harmony that holds together both Irish and English sessions, when the melodic musicians don't have the time . space and vision to do it themselves. It takes a good clear brain to hear: what is going on (OK you know it, you've played it a hundred times before), what can happen here to make it interesting (without buggering up what the others are doing), and what you can do yourself, and finish up alongside the other... without making them look bad. Sessions are easy, and hard. If you're better than the others, the bonus is in YOU to blend in.
Back to fiddling. No difference until recently invented stuff.