Inconsistencies and errors in ages of up to 10 years or more are very common on Irish censuses, marriage and death certificates. Most people didn?t know their ages, they didn?t celebrate birthdays, they weren?t that interested and so they just guessed.
There was no interview to gather the census information. The enumerator left the form at each house and then collected it later, so the information on it is as given by the householder. If it?s wrong, it wasn?t the enumerator?s fault. John Rourke declared he could read and write and he signed both census forms, so I?d say he supplied the information and any errors are down to him. (There can be reasons why someone might deliberately change their age eg vanity or for pension purposes. An an old age pension had been introduced in 1909 for people aged 70 and over, so there was a tendency for some people in their 60s to increase their age in the 1911 census, in the hope of qualifying for a pension, but I don't think either of these reasons apply here). As I say inaccurate ages on censuses were widespread, and there is nothing unusual or sinister about it.
You can verify the correct birth dates of everyone born after 1864 by their birth certificates. Parish records may be available for births prior to that.
Alexander Irvine was born in 1863 in Antrim town and became a Minister living in the US. This extract from his book ?The Chimney Corner revisited? perhaps explains why people often had to guess their age:
?My mother kept a mental record of the twelve births. None of us ever knew, or cared to know, when we were born. When I heard of anybody in the more fortunate class celebrating a birthday I considered it a foolish imitation of the Queen?s birthday, which rankled in our little minds with 25th December or 12th July. In manhood there were times when I had to prove I was born somewhere, somewhen, and then it was that I discovered that I also had a birthday. The clerk of the parish informed me.?