Complex sentence: Through the growing influence of the "Freeman's Journal" of which he was the sole proprietor from 1850, he became a significant figure in Dublin municipal politics, as well as in national politics at an otherwise quiet period of Irish politics up until 1860.
Split into simpler sentences: Through the growing influence of the "Freeman's Journal" (of which he was the sole proprietor from 1850), he became a significant figure in Dublin municipal politics. He was also active in national politics at an otherwise quiet period of Irish politics up until 1860.