Complex sentence: Then aristocrat-dominated, Georgian Dublin was the second most populous city at the time of the formation of the United Kingdom of Great Britain and Ireland in 1801, and was often described as the second city of the UK.
Split into simpler sentences: Then aristocrat-dominated, Georgian Dublin was the second most populous city at the time of the formation of the United Kingdom of Great Britain and Ireland in 1801, and was considered the de-facto second city. As such, it was often described as the second city of the UK.