Welcome to Ireland Reaching Out and apologies for the delay in getting back to you.
In relation to your questions:
(1) In order to find out more about John and Margaret, a good starting point would be to try to find a marriage record - have you already tried to do this using ancestry, rootsireland.ie or familysearch?
(2) The link below shows the dispersal of the family name in 19th Century Ireland:
In order to learn more about the family name, this University College Cork site below is interesting:
(3) The Glens of Antrim Historical Society may be able to give you info on any local or parish history published in the area. Such publications are invaluable as they often uncover names that are not records in other parish records:
I hope a McNeil or someone researching the name gets in touch with you.