A census is when the population is counted.
Long ago a census was taken every decade. Nowadays they are taken every 4 years.
The number of people in any area is listed under the name of the place. It might be listed under a city or under a smaller place within a city.
If you compare the population in a place from one census to another you can work out whether the population of that area has increased or fallen.
You can find out up-to-date information on population figures from the census of population www.cso.ie.
Your local library has the information on older figures.