There are a number of songs called 'Pretty Polly' but if you mean the one that generally starts 'I courted Pretty Polly the live long night/And left her next morning before it was light' then it is a branch of a widespread English song generally called 'The Cruel Ship's Carpenter'. Roud number 15. The US versions I have heard lose some of the narrative (e.g. Polly's return as a ghost) but have that great tune that both Woody Guthrie and Bob Dylan made use of. In terms of date, certainly eighteenth century perhaps seventeenth, without some research I could not be more specific. Other titles include 'The Gosport tragedy: or, The perjured ship-carpenter', 'Molly the Betrayed' and 'Love and Murder'.
It seems to be one of those songs that starts as a wordy broadside ballad but gets whittled down - and improved - on the way.