Complex sentence: Although this is a significant growth over recent years, it is lower than historical figures--in 1841, the population of the 26 counties which would later form the Republic of Ireland was over 6.5 million people.
Split into simpler sentences: Although this is a significant growth over recent years, it is lower than historical figures. In 1841, the population of the 26 counties which would later form the Republic of Ireland was over 6.5 million people.