Kim,
Do you know Kate's maiden name and their religion?
If you have a good idea of the date of their marriage you could apply for a marriage cert. for them, similarly a birth cert for son William.
If John Bunyan Chapman was Church of Ireland then you may be able to get a copy of his parents marriage as non-RC marriages were registered from 1845 onwards.
The parish registers for Roscrea are available on microfilm at the NAI in Dublin and possibly at your local LDS(?). A look at this register would be well worth while as it may reveal siblings and cousins etc.
Contact details for BMD's
www.gro.ieRonnieG