The 1939 Register is a snapshot of live in England and Wales at the beginning of World War II. It was taken on Friday, 29th September, under the National Registration Act of 1939, an Act of Parliament introduced as an emergency measure at the beginning of World War II. The Act also brought in identity cards which had to be carried at all times. It was repealed in 1952 after which it was no longer a requirement to carry identity cards in the UK.
In the United Kingdom, a census of the population has been taken every 10 years since 1801, with the exception of 1941 (although a similar register was taken on 29 September 1939, shortly before the outbreak of war).
In Ireland, the census was taken along with the UK census until 1911. No census was taken in Ireland in 1921 because of the Civil War. The first census taken by the Irish government was in 1926.
Census records are released to the public a hundred years after they were taken, meaning the latest census we can view is 1911 (although the 1939 Register is also available for England and Wales). The next full UK census released will be the 1921 census, which is due to be published on 1 January 2022 (though there is growing pressure for it to be released earlier). The next Irish census released will be in January 2027.
There are a great number of genealogy websites out there. Some offer worldwide coverage, from birth marriage and deaths through census data to passenger lists and newspaper archives. Others focus on one subject or one region, with some focusing on a single town. Many are free and some are either subscription or pay as you go.
Most of these sites are free to search and to view the transcripts of the information. Some require a subscription (or link to a subscription site) to view the original record.