This isn't actually Irvin's first stint with the Lions. Back in 2011, he spent about a month with the Lions as a member of their practice squad. Later that season, he went on to play in four games for the Jacksonville Jaguars, and he appeared in 12 games for the Tampa Bay Buccaneers a year later. Last season, in 2013, Irvin played in six games for the Dallas Cowboys and recorded 3 tackles.
Irvin entered the NFL in 2009 as a third-round pick out of Georgia. He spent a couple seasons with the Carolina Panthers, but he only appeared in a total of two games with them. He then spent some time with the Lions practice squad before jumping around various NFL rosters.
Now that he's back in Detroit, it looks like Irvin could compete for the fourth spot on the depth chart at defensive tackle. The Lions' top three is set with Ndamukong Suh, Nick Fairley and C.J. Mosley, but they could use a fourth defensive tackle, and Irvin seems to have a good shot at filling that spot as of right now.