Complex sentence: The O'Mahonys regained the castle but in July Captain Robert Harvey recovered it, but the previous March the under aged ward of Carew Donal O'Mahony succeeded as chief on his brother's death.
Split into simpler sentences: The O'Mahonys regained the castle but in July Captain Robert Harvey recovered it. The previous March the under-aged ward of Carew, Donal O'Mahony, had succeeded as chief on his brother's death.