Cowboys defensive lineman David Irving has been suspended four games for violation of the NFL's substance abuse policy.The league announced the suspension Friday afternoon ahead of many league officials leaving for summer vacation.Irving will be eligible to return to the Cowboys active roster on Monday, October 1 following the team's September 30 game against the Detroit Lions.Irving is eligible to participate in all offseason and preseason practices and games.The suspension marks Irving's second-straight year opening a season with a four-game suspension.