Shortest Traditional Song? How about "Davie & Jeannie" from the great Gavin Greig Collection. It says a hell of a lot in a few lines:-

Tell me, what was on yer mind, Bonnie Jeannie?
Tell me what was on yer mind when ye changed yer bed for mine?
Were ye tired o'layin; yer lain, Bonnie Jeannie?

It was all for love of you, Bonnie Davie,
It was all for love of you, but I see my folly noo,
For ye've gaed me cause tae rue, Bonnie Davie.