Complex sentence: At the time of the 1841 census the population was 2,122 but by the 1851 census the population had decreased to 1,454 due to The Great Famine.
Split into simpler sentences: At the time of the 1841 census the population was 2,122. However, by the 1851 census the population had decreased to 1,454 due to the Great Famine.