Hello Candice,
It's great to hear you're diving deep into your ancestry!
Before 1864, records were kept in the form of church and parish records. All the surviving records have been sorted into
county-based genealogical centres. As such this might be a good place to start your search on John's early years. In 1864 civil registration was introduced in Ireland, which is when we start seeing the formal registration of births, marriages, adoptions and deaths. These records have been kept at the
General Register Office of Ireland (GRO). Maybe a look at their website could shed some more light on whether your 3x Great Grandparents had more children for example.
Additionally, the
National Archives and National Library offer free genealogy advice in person and via email. They won't do the research itself, but can guide to the best resources.
Wishing you the best of luck on this journey already!
Alex