What age would a minor be getting married in 1878?

I think 'minor' would have referred to under 21, but there could have been earlier marriages before 17 years of age. I know in the country there were arranged marriages. I know of a family who tried this for their daughter aged 16 in the 1960's and she scarpered!!

Under 21 - 21 was 'age of majority'.

Don’t confuse the ‘age of majority’ with the ‘legal age’ for marriage. If you are looking at a Civil Cert and it states ‘Full’ age it means over 21 years at the date (1878) you give.

In Ireland the age of majority was not reduced from 21 to 18 until 1 Jan 1970 (by the Family Law Reform Act 1969). The ‘drivers’ for that change primarily were commercial – legally anyone under 21 was an ‘infant’, could not enter into most contracts, e.g. buy a house, etc. although they could marry.

Legal age for marriage is quite different. In 1904 a male could marry at fourteen and a girl at 12, but marriages at that age were extremely rare. Under Roman Catholic canon law, the age of marriage was raised in 1917 to 16 for males, 14 for females (with a stipulation that marriages were not to be celebrated unless the parties were also of age to marry according to the local civil law). In Ireland the civil law ages were 14 and 12 and remained so until the Marriages Act 1972 raised the minimum age for marriage to 16 years, with a provision enabling the President of the High Court to grant an exemption for a person who has not attained that age. Today the parties being married must be over 18 years of age or have a Court Exemption Order if this is not the case and parental consent is no longer a condition.