JEAN RITCHIE was making a guess and it was wrong, bally in irish means place of in scottish gaelic pretty is breagha, so both are incorrect, the thing to do hear is change bally to something like this
I wish I were in London
Or some other seaport town,
I'd set my foot on a sail boat
And I'd sail them seas all around.
I'd sail them seas all around,
I'd sail them seas all around,
I'd set my foot on a sail boat
And I'd cross/sail them seas all around,

bally in englsh slang means damn

please no more uninformed incorrect guesses,
as far as i am concerned the word bally is used by upper class english aristocrats like lord peter wimsey, this sort of conversation, between lord peter and his servant

Bunter can you get me a bally boat, yes your lordship would you like your bally boat to have a cock tail cabinet