There are two distinct peoples in Ireland, who see and define themselves differently and antagonistically, the Catholic “Irish-Irish” Nationalists and the Protestant “British-Irish” Unionists.
Ireland, which had been ruled by England since the 12th century, was partitioned in 1920-21 into Six and 26 Counties entities. The border dividing the Six and 26 Counties does not coincide, or even approximate to, the geographical location of the two distinct Irish peoples/ identities. It cuts right through them.
There is a large Catholic-Nationalist population in Belfast, but broadly the Protestant