The Dirty Rat is a Dwarf pub in Ankh-Morpork. Its precise location is not specified in Raising Steam, although it might be a safe bet to suppose it's in the Cable Street area. With the covert aid of the grag Ardent, it becomes a hot-bed of drink-fuelled disaffection. At least, until a barman whose antennae are twanging with the word trouble calls time.
The Rat is not listed in the public house and tavern directory of The Compleat Ankh-Morpork. Perhaps as a dwarf bar it slipped under the radar, or was beneath notice, or something... the text notes that there was another Dwarf bar a few streets away.