I found the full marriage record for your grandparents, John Sullivan and Dympna Hardiman, as well the birth index for your dad, Joseph C. Sullivan.
Before sending this information I'd like to do a little more research to see what else I can find about your dad and his ancestors in other records, so that I'll have more complete information to send you.
For example, I want to see if I can find Dympna in the 1901 and 1911 census enumerations, as these census returns should give the names of her parents, if living, as well as the names of any brothers and sisters she had. I'll also want to see if I can locate the marriage record for her parents.
Finding records about John Sullivan will probably be more challenging, as Sullivan is one of the most popular names in Ireland.
I hope you don't mind waiting a little bit longer for this information, but I should be writing back, hopefully by this weekend, or sometime next week, after the research is completed.
With Kind Regards,