Complex sentence: On the death of his father Piaras in 1431, Cormac succeeded to the post of Prior of Drumlane due to hereditary succession rights, rather than to his experience, as he would have been in his early twenties at the time.
Split into simpler sentences: On the death of his father Piaras in 1431, Cormac succeeded to the post of Prior of Drumlane due to hereditary succession rights. He would have been in his early twenties at the time.