The geographical remit of the diocese covers half of the City of Belfast to the east of the River Lagan and the part of County Armagh east of the River Bann and all of County Down.

Later, by decree of the Irish Parliament, a similar new body became the state church in the Kingdom of Ireland.

Over the centuries, numerous dioceses were merged, in view of declining membership.

In 2011 the arms of the united diocese were confirmed by the office of the Chief Herald of Ireland.

[2] The arms are blazoned as follows:[3] Quarterly, 1st and 4th for Down: Azure two keys in saltire, the wards upward Or suppressed by a lamb passant Argent; 2nd and 3rd for Dromore: Argent two keys in saltire the wards upward Gules suppressed by an open book Proper between in chief and in base two crosses patée fitchée Azure.The Diocese is the second largest of the Church of Ireland in terms of church population, with around 91,000 people and more than one hundred serving ordained Clergy.