Fedayeen would be Regular warfare (since they're "uniformed troops") using asymmetric tactics, I guess.
It's really like trying to distinguish between strategic attack and interdiction...if the troops haven't started moving, it's the former, if they're moving to the front line it's the latter.
So if the Fedayeen are not acting as part of a national force on force, it's Irregular?
Hell, I don't know.