What's the difference between tioraidh and mar sin leat?
tioraidh is less formal.
It is like bye (tìoraidh) and goodbye (mar sin leat/leibh)
I was told recently that "mar sin leat" is only a reply to someone saying goodbye, e.g. "slàn leat" or "beannachd leat".
To an Irish speaker "mar sin leat" sounds very odd by itself.