Complex sentence: Like Grattan, Guinness was publicly in favour of Catholic Emancipation from 1793, and, like most of the population, he was not a supporter of the anti-sectarian United Irish during the 1798 rebellion.
Split into simpler sentences: Like Grattan, Guinness was publicly in favour of Catholic Emancipation from 1793. However, he was not a supporter of the United Irishmen during the 1798 rebellion.