Complex sentence: In the same year the town sold its customs rights-which ran from Groomsport in County Down up to Larne in County Antrim-to Belfast and this in part led to its decline in importance as the province of Ulster grew.
Split into simpler sentences: In the same year the town sold its customs rights-which ran from Groomsport, County Down up to Larne, County Antrim to Belfast. This in part led to its decline in importance as the province of Ulster grew.