It was a requirement to register full details of a child’s birth within 21 days. Failure to do so invoked a fine and in rural areas particularly, the 21 day deadline for registering births was frequently disregarded. In order to avoid payment of the fine the dates of birth were adjusted to comply with the deadline. Where Church records give the dates of birth these are generally more accurate than those given in civil records; however, variations of up to nine months can occur. Please contact the county genealogy centre from which the record came if you have a query about a particular record.
above compliments of an excellent search site
http://www.rootsireland.ie