The NICRA sought to prevent discriminatory allocation of government jobs and council housing. The victims of such discrimination were the Catholic/Republican minority - at least, that's how that minority would be construed during the Troubles, once religious differences became convenient labels for the political factions. But how was such discrimination practised before that? For example, how was the oppressed group defined, and how were people identified as members of it?
While Northern Ireland does not divide cleanly into "Catholic" and "Protestant" communities, quite a lot of the population belonged by heritage to one or the other. And you can tell them apart by their accents.
Northern Irish Catholics sound a lot like natives of Ireland. Northern Irish Protestants sound much more like the inhabitants of south-west Scotland. Both accents seem to be quite hard to shed; a friend of mine who moved to London over thirty years ago still has recognisable traces in her voice. The accents have been perpetuated through segregated schools, and as markers of identity.
It's also the case that many neighbourhoods are populated only by one or the other group, which tends to strengthen the group identities. That also made gerrymandering of the electoral districts easy for the Protestant-controlled government of Northern Ireland before the Troubles started. For more details of the discrimination, the Wikipedia article on Northern Ireland Civil Rights Association has decent coverage.