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I assume your Costelloes were Roman Catholic? The RC records for Freshford go back to the late 1700s and there don't appear to be any gaps in the records. Here is the link to the parish register https://registers.nli.ie/parishes/0985 I searched the subscription site Roots Ireland and could not find a Michael Costelloe baptismal record around 1821 with father John and mother Margaret.I also searched for any children of the couple with no success.
The 1826 Tithe listings are available for Freshford civil parish but I only found one Costello/Costelloe record-- a Patrick in Knockroe townland.
Any other clues?