No idea if this has been mentioned, but the Umbers of Last Hearth could possibly be a reference to Northumberland, the north-eastern region of England which the North in ASOIAF is partly based on. Also, the fact that they are in the very north-east of the North and close to the Wall makes it representative of a city such as Newcastle-upon-Tyne, a north-eastern English city which is close to Hadrian's Wall.