Using census records
In the UK, a census has been taken every 10 years since 1801. The first four censuses (1801-1831) were mainly headcounts and contained little personal information.
The 1841 Census, conducted by the General Register Office, was the first to record the names of everyone in a household or institution. However, their relationship to the head of the household was not noted, although sometimes this can be inferred from the occupation shown (eg servant). Those under the age of 15 had their proper ages listed, but for those who were older the ages were supposed to be rounded down to the nearest five years, although this rule was not strictly adhered to. Precise birthplaces were not given - at best the birthplace can be narrowed down to the county in which the person was living.
From 1851 onwards the census shows the exact age and relationship to the head of household for each individual; the place of birth was also listed, but with varying degrees of precision.
The censuses are reasonably accurate. However, ages in particular are frequently shown incorrectly, though often the difference is only one year; in general the younger the individual the more accurate the age shown. Older people have a tendency to lie about their age!
Birthplaces often vary from one census to the next: a common error is to show the place where the census was taken as the birthplace, but most of the variations in birthplace can be accounted for by changes in geographical scale (for example, the nearest town being shown instead of the precise village, or a city being shown instead of the relevant suburb).
The censuses are also remarkably complete - though inevitably a small percentage of the population wasn’t recorded for one reason or another, and in some cases the records are missing or damaged (notably in 1861).
urthermore, all censuses of Ireland before 1901 have been lost or destroyed.
Because of World War II, there was no census in 1941. However, following the passage into law (on 5 September 1939) of the National Registration Act a population count was carried out on 29 September 1939, which was, in effect, a census.
In the UK there is a privacy law, which means the census returns are not made public for 100 years. Therefore the latest census we currently have access to is 1901 and this is the first one you should try to access.
Which of your ancestors were alive in 1901, how old would they have been and where were they living? If you have any of these details, you should be able to locate your ancestors on the census.
There are websites where you can access the census returns online.
Alternatively you can view the returns for an area from the county records office.