You can search for his birth on the LDS pilot site:http://search.labs.familysearch.org/recordsearch/start.html#...
If you find it, you can order a photocopy of the certificate from GRO Roscommon for €4. www.groireland.ie/
You may be able to find him in the 1901 or 1911 Irish censuses:www.census.nationalarchives.ie/search/?then
(All earlier Irish censuses have been destroyed).
If you haven't got your gt, grandad's father's name, you should be able to get it from his English marriage certificate.