Complex sentence: Following the arrival of 12,000 Catholics fleeing famine in Ireland in the 1840s and more immigration thereafter, St David's was built as Cardiff's principal Catholic church in 1887, and it became a cathedral and seat of the Roman Catholic Archbishop of Cardiff in 1916.
Split into simpler sentences: Following the arrival of 12,000 Catholics fleeing famine in Ireland in the 1840s and more immigration thereafter, a Catholic church was built in Cardiff. It was Cardiff's principal Catholic church, and it became seat of the Roman Catholic Archbishop of Cardiff in 1916.