You were a minor until age 21 at that time. I don’t think there was a statutory minimum age for marrying in Ireland (England was different as a result of Hardwicke’s Marriage Act) but in general marriages under 18 in Ireland were pretty rare.
The age discrepancies that you have identified tend to come from census and death certificate information where the ages are routinely out by anything up to 10 years. Few people celebrated birthdays in Ireland in the 1800s, many had no birth or baptismal certificates and so didn’t always know their own ages accurately. I dare say they did lie about their ages sometimes but in general most neither knew their age all that accurately nor cared.
Alexander Irvine was born in 1863 in Antrim town and became a Minister living in the US. This extract from his book “The Chimney Corner revisited” perhaps explains why people often had to guess their age:
“My mother kept a mental record of the twelve births. None of us ever knew, or cared to know, when we were born. When I heard of anybody in the more fortunate class celebrating a birthday I considered it a foolish imitation of the Queen’s birthday, which rankled in our little minds with 25th December or 12th July. In manhood there were times when I had to prove I was born somewhere, somewhen, and then it was that I discovered that I also had a birthday. The clerk of the parish informed me.”
