I hope Sillivan is a typo or a name used after arrival in Australia as there are none listed.
The marriage usually took place in the bride's parish, in this case Athlone as no Sullivan's reside there. In the
cencus sustitute Griffith Valuation there are Sullivan's
Henry Hill, Parish of St. Mary Athlone tenant of Laurence
Kelly. #27 Tangier Lane off Mardyke St. - a house & yard
John Sullivan, Parish of Piercetown, TL
tenant James Higgins, #4a house and garden 1 acres 0 shllings and 10 pence
There are 3 Sullivan still on telephone for Athlone and 9 Hill's, 1 in Athlone, you need to contact them. Addresses given are sufficient for mail delivery.
There is a not for profit Heritage Foundation which will have parish records for the family, not sure if will go back to such early dates as mentioned but the children should be there. Athlone records start 1810 and
You have both names of the parents so should not have any trouble.