Complex sentence: In economic terms, the Irish Catholics were nearly at the bottom in the 1850s; however they reached the national average by 1900; and by the late 20th century they far surpassed the national average.
Split into simpler sentences: In economic terms, Irish Catholics were nearly at the bottom in the 1850s. They reached the national average by 1900, and by the late 20th century they far surpassed the national average.