Original sentence: While the Ulster Catholics of Northern Ireland have long opposed Northern Ireland's existence, the Ulster Protestants of the three Republic of Ireland counties have assimilated well (although there was a good deal of migration into Northern Ireland's six counties following partition)
Edited sentence: While the Ulster Catholics of Northern Ireland have long opposed Northern Ireland's existence, the Ulster Protestants of the three Republic of Ireland counties have assimilated well (although there was a good deal of migration into the six counties following partition)