Folk tunes dont come equipped with harmonies, you have to provide them yourself according to what suits your ears. However, as has been pointed out, the harmonies are remarkably strongly implied in the Drunken Sailor. With a firsst bar conatining only EG and B, and a second bar containing only D F# and A, you'ld have to be pretty avant-garde not to use Em and D chords. The overall harmonic structure of Drunken Sailor
Em ///
D///
Em///
D/Em/
is extremely common in fiddle tunes in the 1700-1900 period, both in Ireland and Britain.Sometimes in the minor(Dorian as here, or Aeolian) also in major (substitute G and D chords say), or the Myxolydian (substitute A and G). You could write a book about tunes with this structure and its possible variations).