Complex sentence: Clery was brought up to a considerable extent by his relative Charles Dawson (baker, former MP and Lord Mayor of Dublin) and his cousin William Dawson (who used the pen-name "Avis") became his closest friend and associate.
Split into simpler sentences: Clery was brought up to a considerable extent by his relative Charles Dawson (baker, former MP and Lord Mayor of Dublin). His cousin William Dawson (who used the pen-name "Avis") became his closest friend and associate.