Why are there sometimes discrepancies between the dates on church and civil records?

This may help in part to explain why a child was "BAPTISED BEFORE HE/SHE WAS REGISTERED AS BEING BORN" an impossible task in logic !.

There may also be a connection (legal requirements) why the age discrepancy in the 1901 and 1911 Census of Ireland was quite common.
I have found that in general the 1911 details were more accurate.

Information regarding births, marriages, and deaths was not always recorded and, even when it was, it was not always written down accurately, especially in birth and death records. Approximately 15 per cent of births were not registered, especially in the earlier period.

Some people remained unregistered due to the failure of their parents to register them at birth and others who nominally complied with the law made incorrect entries. Much of the work necessary to ensure more complete coverage was completed during the period 1879-1900 when Thomas Grimshaw was Registrar General. The Births and Deaths Registration Act ( Ireland ) 1880 set out the procedures to be followed and the persons who were required to give information to the registrars in respect of births and deaths. It laid down time limits for persons to comply with the regulations and provided a system for the correction of errors. Provision was also made for the appointment of assistants to registrars and superintendent registrars and penalties designed to protect the integrity of the records form fraud were introduced.

Most interesting thoughts, John! (reading between the lines in old records IS revealing :-) )

"Adjusted" ages were used in the 1911 census to ensure qualification for the old age pension which was being introduced about that time. That is what I have heard. This would indicate that the 1901 dates were more reliable??

I stand corrected and and upon checking my own
Know data I can confirm that you are indeed correct.
Sorry if I gave wrong info to anyone and hindered
Rather than helped which is my only intention.
Thank you Michael for putting the record straight.

Im not 100% sure, but I heard that some british forces checked some of the census records so for that reason some people lied if they were involved in rebel activities!

In the Catholic church the baptism would be as soon as possible for the religous because the number of infant deaths was high. Anyone not baptised would not be admitted to heaven. On that basis the baptism date should be more accurate but the priest would record the date later - but only for Catholics. Non-catholics (many in Ireland) are still being transcribed and it is possible to find them on websites. None of this covers those that could not be bothered to register with the church or were too poor to be bothered.

With the census you can have big differences of ages for reasons already given but also that people were not required to know their age as we do today. In my experience the age can vary ten years between the two census. Also kids ages are often worked out from the youngest or eldest and then everyone else is calculated from there - I have seen this in my own relatives in Ireland today. Age values at death are equally difficult and can easily vary by five years.
So, unless your birth date census and death ages agree you will always be wondering.

The subject is worth a long article sometime because it covers a lot of human behaviours not the time accurate records we have today.
Having said that , I was living without a birth certificate for over 60yrs! :)