You need to contextualise your query according to the circumstances of the time.
Whilst early 20s would be a more common age to marry, your great great grandmother would have been at prime marrying age at the height of the Napoleonic war / War of 1812, when a significant number of young eligible Irishmen would have been serving abroad. So, just like the "maiden aunts" of the first world war generation, there would have been a surplus of marriageable young women over marriageable young men. Marrying somewhat later in life would not, therefore, be that uncommon a thing.