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Surnames
Irish surnames were more susceptible to change than others. In the last century most Irish people spoke the Irish (Gaelic) language and many emigrants were unfamiliar with a written version of their surname in English. Church, State and Census were recorded in English and the spelling and translation of surnames was haphazard, so don't be put off if you discover that your Irish ancestor's name is different from the way you spell it now.
Ages and Dates
Ages and dates of birth given on records and gravestones are often approximate to within ten years either way. In most families a child was born every year or so and the exact birth dates of each were seldom remembered. As the child got older, its precise year of birth was often forgotten, the child grew up without knowing exactly how old he/she was, and his/her age was often understimated. Once a person was visibly old, his/her age could often be overestimated. Therefore, an age can generally be taken to be correct to within ten years wither way. Dates of birth were frequently made up upon coming into contact with the first official who recorded it.