it was a guess by jean ritchie,
the word bally in irih gaelic does not mean pretty, in scottish gaelic pretty is breagha, so neitrher fit the bill, jean ritchies guess was just wrong.
Jean clearly did not know what the word meant, the sensible thing to do here is is sing, set my foot on a sailing boat
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 sail them seas all around