Cowboys defensive tackle David Irving will miss his third consecutive game with a concussion. He has not cleared protocol. Irving, who has seven sacks in eight games, is the only player Dallas has ruled out for Sunday’s game against Seattle. But the Cowboys list six players as questionable, including left tackle Tyron Smith. Smith, who missed two games earlier this season with a groin injury and has battled a back issue much of the year, returned to practice on a limited basis Friday. He will try to play through his back and knee injuries.