The Rootes are said to be lords of Harroway. I presume this means that the Harroways were of Harroway -- giving their name to it -- before they were raised to the station of lords of Harrenhal, and the Rootes took up the rule of Harroway castle.

Ah.... not quite... there's a town on the Trident, a little upriver from the ruby ford, called Lord Harroway's Town... it's called Harroway for short, since the full name is a little much... that's where the Rootes have their lordship. We get a brief glimpse of LHT in ASOS...