Unionist v Nationalist
In Northern Ireland all elections are a sectarian headcount because there is one question that trumps all others - 'Should there be a United Ireland or should Northern Ireland remain part of the UK?'. If you are a Catholic or from a Catholic background you will vote Nationalist and if you are from a Protestant background you will vote Unionist. The following graphs show how Westminster elections in Northern Ireland since 1983 have seen more nationalist and fewer unionist MPs. This is not because more people in Northern Ireland have been convinced of the benefits of the Nationalist argument. It is because demographics have seen the proportion of Catholics to Protestants move in favour of the Nationalists.
The 2015 election was the first election since 1983 that saw the proportion of unionist MPs increase. This was due to electoral pacts between unionist parties and apathy amongst nationalists. Without these two influences I think this election would result in an equal number of unionist and nationalist MPs returned to Westminster.
The Good Friday Agreement resulted in a hardening of sectarian voting
The 1998 Good Friday agreement ironically resulted in a hardening of sectarian voting, the more moderate UUP and SDLP were replaced by the more sectarian DUP and Sinn Fein.