Complex sentence: During this time the chief of the O'Neills who was called The O'Neill, were inaugurated at the fort, the clan seat of the O'Hagans, hereditary stewards to the O'Neills.
Split into simpler sentences: During this time the chief of the O'Neills who was called The O'Neill. Chiefs were inaugurated at the fort, the clan seat of the O'Hagans, hereditary stewards to the O'Neills.