One of the first things that a researcher needs to remember is that our ancestors were not too bothered about their exact ages. They didn’t have a twenty first, a fortieth or fiftieth. Believe it or not, they didn’t have birthday parties, hen nights or stag nights either. They were too busy trying to eke a living out of the bit of land they had or working for long hours for someone else.
The system of registration of births in the 19th century was that the individual was responsible for registering the birth. The authorities, as an ‘encouragement’ to do this, would impose a hefty monetary fine on the individual who didn’t comply with the law. So a birth registration might take some time to be recorded and the exact day, and place might not be correct when it was finally registered. In quite of a lot of cases, particularly in rural, inaccessible country, the birth was never registered. On the other hand Catholics were encouraged (without the fine) and generally obeyed their clergy, by getting the child baptised as soon as possible after birth. This did not guarantee correct baptismal records though. Sometimes the individual priest, who didn’t have a presbytery, housekeeper or even in some cases, a church, would write the details on a piece of paper for later transcription, in Latin usually, to the Parish Register.
A census of the Irish population was taken every ten years from 1821 until 1911 and manuscript returns for each household survive for all 32 counties for 1901 and 1911. The returns are arranged by townland in rural areas and by street in urban areas. No manuscript returns survive for 1861, 1871, 1881 and 1891 but there are some returns for 1821, 1831, 1841 and 1851 covering some counties, but not Kerry. (Ref: NLI) Even in these Census returns, people used to write round numbers, 20, 30, 50 etc. John Grehan in his excellent book ‘Tracing Your Irish Ancestors’ says ‘The actual date of birth is almost always well before the one reported, sometimes by as much as fifteen years’
When Ireland introduced the Old Age Pension in 1908 many applicants had no birth certificate as Civil Registration did not begin until 1864. So the 1841 and 1851 census returns were used to prove age. An index to the 1841 and 1851 census returns that appear in the Old Age Pension claim forms (T550) is available to consult on microfiche in the Public Record Office of Northern Ireland while there are also some other copies made from the returns of 1821-185, miscellaneous copies and certified copies available to consult in our Reading Room (National Library of Ireland).
You will understand now why your ancestor’s age on the 1911 Census probably doesn’t match up with the age they had given in the 1901 Census. They are probably a lot older in 1911 and not ten years’ older as you would expect. If you were trying to qualify for the Old Age Pension and you had only a vague idea of your date of birth, you would want to be as old as possible, wouldn’t you?