Cowboys star David Irving -- who was just hit with ANOTHER suspension for violating the NFL's substance abuse policy -- says it's "bullsh*t" the NFL won't let him smoke weed.
25-year-old Irving was a stud for Dallas the last four seasons when he was actually on the field ... logging 12.5 sacks in just 10 starts -- but, the dude just couldn't stay out of trouble.
He was suspended for 4 games to start BOTH the 2017 and 2018 seasons ... and just last week, the league announced he's been banned once again.
Now, the guy is speaking out on social media .