Irtok was a planetary system near Ferenginar.
When, in 2374, Rom floated the possibility that the Dominion had conquered Ferenginar, Major Kira Nerys countered that it could not have been taken without first conquering systems like Irtok and Clarus. Misunderstanding this response, Rom assumed Irtok had been taken over by the Dominion, too but, as Worf pointed out moments later, there had been no reports of Dominion activity in or near that system. (DS9: "Profit and Lace")
According to the Star Trek: Star Charts ("United Federation of Planets I"), Irtok was located in or near Ferengi space, in the Alpha Quadrant. This was a single star system. Primary was a Class G star with a magnitude of +5, which was the same brightness as Sol.