I am descended from Catherine Callan of Dundalk, County Louth.But she is a generation or three before your g-grandfather John Callan.She married James Cochrane and they left for America in the 1840's.
I found an entry for your g-grandfather John Callan on the Callans from County Louth web site: http://www.falgen.org/callan/http://www.falgen.org/callan/
Look in the "callan ancestors" section and you'll find some information on John, his wife and 2 sons.Nothing about his parents or siblings appears there, but it seems to me that he could be listed under another John Callan as well.Do you know a birth date, Christening date or death date for him?
The person who is responsible for this web site is Donna Walcovy.Her e-mail address (I last used it to communicate with her in July 2002, so it may no longer be any good) was:firstname.lastname@example.orgHer worke-mail address was:email@example.com .
Sorry I don't have any more information on Callans.I'm frustrated myself in finding information on my ancestors.