I checked the subscription site Roots Ireland and found four baptismal records for children of Thomas Sullivan and Ellen Carey in Kildorrery RC parish. For all four records, the home place was shown as Rockmills in St. Nathlast civil parish. I did not see a record for William, John and Mary but found records for Thomas and Bridget? I assume I have the correct family. Also, there is a tenyear gap between the marriage and the first baptismal record I located. Kildorrery baptismal records are available back to 1824 https://registers.nli.ie/parishes/0065 Maybe William John and Mary were baptized in a different parish which no longer has records back to the 1830s.
Ellen January 7 1841
Bridget February 11 1844
Thomas June 22 1845
Ellen April 25 1854
Let me know what questions you have