It was necessary to pay a fee to register a birth. This was beyond the means of many poorer families and as such a birth could go unregistered.
You were fined if a birth was registered late. In order to avoid the fine, some parents gave an incorrect date of birth for the child so that it fell within the period of registration. This means that a child that was 6 months old could be given a date of birth making them only 2 months old.
It was the responsibility of the parish priest or minister to register marriages. In some cases the priest failed to register the event.
It is estimated that up to 30% of deaths in Ireland went unregistered, even in the 20th century. It is not uncommon to find a death was not registered with the civil authorities.