Hi there!
Not sure on siblings though there must have been some. I have him living in House 4, Ardgroom Inward, Kilcathrine, Cork in the 1911 Census. They had 2 sons and 4 daughters and it is through their first son, John Sullivan (born about 1892) that I am connected. The fact their first son was called John may suggest he had a brother called John but a bit of a long shot!

Going any further back is proving a bit vague but have found a Baptismal record for Daniel Sullivan n 1855 in Dingle. parents listed as Patrick Sullivan and Ellen (?) but no further info.

If any of that helps then let me know.

Cheers!