The name is very common. I looked at the 1901 Irish census and there were 769 people named John O’Neil of whom 33 lived in Co. Down. The name would have been almost as common in the mid 1700s. The other problem you face is that very few records exist in Ireland for the 1700s. Few parishes have church records for someone born around the 1740s.
You have posted this query under the parish of Kilbroney. I don’t know if that was a random choice but if not, the oldest parish records for it are the Church of Ireland which start in 1812, the Presbyterian records start in 1851 and Roman Catholic in 1808. That’s fairly typical of what to expect across the rest of the county.
I am afraid I don’t hold out any real hope of finding any record of John and his family in Ireland.