I suppose it depends whether Hugh continued to have an active link with the Rocktown O'Neills after his marriage to Mary Irons. If not, he might not appear in the 1830's-onwards records, but if he *did* then his name might make an appearance, for example as a sponsor for baptisms or a witness for marriages.
One tip based on my own research; when looking at early church records for the area, it's worth searching for 'Termoneeny' which was another, older name for the parish. 'Greenlough' is another place it's worth searching in - it's another nearby parish and prior to the construction of the R.C. church at Lavey it seems a lot of local O'Neills were baptised/married there, especially the Ballymacpeake Stafford-O'Neills.
Regarding Hugh's son John, what do you know about him? Who did he marry and where? It might be worth trying to widen the search and pick up on other details. Same with Mary Irons, where did she come from? Bear in mind that if she wasn't a Catholic prior to her marriage, she may be recorded in church records, dependent on her prior denomination.