Steven:
Welcome to ireland Reaching Out!
I checked the subscription site Roots Ireland and they did not have a John Cassidy/Campbell marriage anywhere in Ireland. More than likely they were married in a church that no longer has records back to 1827 or earlier.
If John and Susanna left Ireland, you may want to add their story to our XO Chronicles site. Possibly someone will recognize the family.
https://www.irelandxo.com/ireland-xo/history-and-genealogy/ancestor-dat…
Roger McDonnell