The Thompson-Woodcock tree on ancestry has the marriage in Youghal, Cork.
rootsireland.ie has a record of the marriage of William Sullivan to Margaret Geary in 1828 Youghal (R C)
The record will give you the parents' names.
If you haven't used rootsireland.ie before just google it & register the search using the criteria you gave:
Marriages Sullivan William Geary Margaret 1823 + - 5 Cork only checked and you'll get it.
rootsireland is a pay site so the record will cost (but there's a sale on at the moment)