Originally Posted by
luther
Dublin was always seen as the most "English" city in Ireland by provincials and this was coined as a term of derision stemming from the English flag, the Union Jack, by adding the diminuitive, -een.
Another story is that when English royality paid visits to Dublin back before independence some Dublin people apparently waved the Union Jack as the royality passed, much to a lot of the countries annoyance. So the jack part evolved into jackeen.