Complex sentence: He took Dublin and with a Jacobite army of Irish Catholics and Frenchmen, he marched north, arriving at Londonderry on 18 April, 1689 and summoned the city to surrender.
Split into simpler sentences: He took Dublin and marched north with a Jacobite army of Irish Catholics and Frenchmen. The army arrived at Londonderry on 18 April, 1689 and summoned the city to surrender.