Complex sentence: O'Donnell opposed this compromise and took the anti-Treaty side in the ensuing Irish Civil War, being among the anti-Treaty IRA men who took over the Four Courts building in Dublin and helped to spark the outbreak of civil war with the new Free State government.
Split into simpler sentences: O'Donnell opposed this compromise and took the anti-Treaty side in the ensuing Irish Civil War. He was among the anti-Treaty IRA men who took over the Four Courts building in Dublin and helped to spark the outbreak of civil war with the new Free State government.