Who is Paul Irvin from Without Remorse and what is their importance?

Master Gunnery Sergeant, Paul Irvin, is a Marine and volunteer at Boxwood Green. He recognizes Kelly from his tattoo, and he explains that he was a back-up for operation Plastic Flower. After explaining why the chopper was late, Paul tells Kelly he would have reacted the same way Kelly did. He questions Kelly about their mission, and Kelly gives him information about what and why it is important.