located_in_the_administrative_territorial_entity a55 road 
___MASK43___ (; Ulster Scots : "") is a top - level constituent unit of the United Kingdom in the northeast of Ireland . It is variously described as a country , province , region , or " part " of the United Kingdom , amongst other terms . ___MASK43___ shares a border to the south and west with the Republic of Ireland . In 2011 , its population was 1 , 810 , 863 , constituting about 30 % of the island ' s total population and about 3 % of the UK ' s population . Established by the ___MASK43___ Act 1998 as part of the Good Friday Agreement , the ___MASK43___ Assembly holds responsibility for a range of devolved policy matters , while other areas are reserved for the British government . ___MASK43___ co - operates with the Republic of Ireland in some areas , and the Agreement granted the Republic the ability to " put forward views and proposals " with " determined efforts to resolve disagreements between the two governments ". The ___MASK46___ Lagan ( ___MASK92___ Scots : " Lagan Wattèr ") is a major river in ___MASK43___ which runs 53 . 5 miles ( 86 km ) from the Slieve Croob mountain in County Down to Belfast where it enters Belfast Lough , an inlet of the Irish Sea . The ___MASK46___ Lagan forms much of the border between County Antrim and County Down in the east of ___MASK92___ . It rises as a tiny , fast - moving stream near to the summit of Slieve Croob ; Transmitter Road runs nearby . From here it continues on its journey to Belfast through Dromara , Donaghcloney and Dromore . On the lower slopes of the mountain , it is joined by another branch from Legananny ( Cratlieve ) Mountain , just opposite Slieve Croob . At Dromara , about four miles from its source , its height above the sea is 390 ft ( 119m ). As the river continues on its journey to Belfast , it turns east to Magheralin into a broad plain between the plateaus of Antrim and Down . The ___MASK98___ ___MASK87___ Great Britain and ___MASK43___ , commonly known as the ___MASK98___ ( UK ) or Britain , is a sovereign country in western Europe . Lying off the north - western coast ___MASK87___ the European mainland , it includes the island ___MASK87___ Great Britain ( the name ___MASK87___ which is also loosely applied to the whole country ), the north - eastern part ___MASK87___ the island ___MASK87___ Ireland , and many smaller islands . ___MASK43___ is the only part ___MASK87___ the UK that shares a land border with another sovereign statethe Republic ___MASK87___ Ireland . Apart from this land border , the UK is surrounded by the Atlantic Ocean , with the North Sea to its east , the English Channel to its south and the Celtic Sea to its south - south - west , giving it the 12th - longest coastline in the world . The Irish Sea lies between Great Britain and Ireland . With an area ___MASK87___ , the UK is the 78th - largest sovereign state in the world and the 11th - largest in Europe . It is also the 21st - most populous country , with an estimated 65 . 1 million inhabitants . Together , this makes it the fourth most densely populated country in the European Union . ___MASK7___ is the capital and largest city ___MASK87___ ___MASK43___ , the second largest on the island ___MASK87___ Ireland , and the heart ___MASK87___ the tenth largest Primary Urban Area in the United Kingdom . On the River Lagan , it had a population ___MASK87___ 286 , 000 at the 2011 census and 333 , 871 after the 2015 council reform . ___MASK7___ was granted city status in 1888 . ___MASK36___ ( ___MASK92___ - Scots : ) is an island in the North Atlantic . It is separated from Great Britain to its east by the North Channel , the Irish Sea , and St George ' s Channel . ___MASK36___ is the second - largest island ___MASK87___ the British Isles , the third - largest in Europe , and the twentieth - largest on Earth . A Primary Urban Area ( PUA ) is an area defined by the Department for Communities and Local Government in the ___MASK98___ as a statistical tool for analysing the major cities ___MASK87___ England , originating as part ___MASK87___ their " State ___MASK87___ the English Cities " report and database . The A55 ___MASK25___ forms ___MASK7___ ' s outer - ring ___MASK25___ .