It seems to me that the chorus at least did originate in Belfast. The line "she is the belle of Belfast city" loses its punning impact when sang about another city. Whoever wrote the song had a wonderful sense of alliteration and composition .It's such a simple singable song ,but very cleverly written , isn't it? I see the phrase "belle of belfast city" as integral to the chorus and therefor probably not adapted from another source city .Certainly not from that Welsh town with the long name.