It's very common for ages in marriage register entries to be wrong - but the only instances I've come across of the wrong date being given were where the vicar wrote the wrong year. It would be interesting to see some of your Irish examples as my experience is mostly of English records.
I didn't express myself very well, note to self...........
I meant number of years married, not wrong marriage dates, sorry.
It's mostly in the censuses where the wrong age is given and not only by people who cannot read or write. I had great 'fun' with the family in the census attached.
James Foye was the father of Patrick Malley's first wife Mary Foy who died in 1897.
When Patrick married Mary Foy in 1883 his declared age was 27.
In the attached 1901 census Patrick's declared age is 40 which is inconsistent with the marriage record, he should have declared 45.
In the 1911 census Patrick's declared age was 58.
In Patrick's death record he is a widower age 75, he died in 1936 and would actually have been abt 80. For interest sake and as an example of an Irish death registration record, it is attached, Patrick is entry number 2.
Patrick's first marriage to Mary Foy is also attached and you will see there isn't any discernible difference from the English parish registers.
About Irish birth records.
I would not be able to find an example right now but I do know there are instances in my tree where the birth date in the registration is later than the date of baptism.
Michael, in the attached census age 15 is entry number 42 in the birth record for 1885, attached