You haven’t given any dates so it’s difficult to comment in detail but the further back you go the less reliable certificates are. In the 1800s people in Ireland didn’t generally celebrate birthdays and often didn’t know how old they were. So ages were often guessed. With marriages there might be other reasons to give false information eg saying you were over 21 to avoid needing parental consent. And with death certificates the information obviously is second hand and so guessing the age was the norm. I found one recently where a woman was given as 63 but her correct age (as per baptism and marriage) was 83. So 20 years out. No proof was required. The informant’s information was normally taken at face value.
