There's a concept called the Hajnal Line. It is usually used to demarcate an area of Western Europe, where mothers married relatively late and had relatively fewer children. Ireland is usually placed outside this territory, but it is somewhat debated, since the earlier census records don't really survive and a lot of parish records don't go back far either.
Some certainly had a lot of children. My grandfather was one of sixteen; his mother was 22 when married. Daniel O'Connell supposedly was one of twenty-two.
I've heard that pre-famine Ireland was different and younger marriages more common. Most of my experience is post famine. Most female ancestors seem to have been 25-30 when married.
Oldest 33. Youngest 17 (said she was 18, her father died when only a few years old)
Frankly, I would be skeptical that the average 12-year-old girl would have been fertile at this time in Ireland. No doubt, this has since changed, due to better nutrition and less disease. Not that, I would say none would have been, just the average, as a guess.
My own personal thought would be that a 12-year-old bride would have been extremely rare. I base this on reading a few years of pre-famine banns in Kerry, specifically Glenbeigh (a very mountainous country place, opposite the bay from Dingle) which does not have many surviving marriage records, only about 5 years, from 1830-1835.
To summarize: no first cousin marriages (this is what makes it seem unlikely to me). Some second cousin, though relatively few. Most consanguineous were second cousin-once removed or even third cousin, hard to tell which was more common.
Reported ages were often wrong and sometimes wildly wrong. Usually a "0" (as in 50 or 60) would indicate a significant error, very easily 4 or 5 years, but even possibly more.
