During Mr Irvine's rugby coaching era, Collegiate 1st XV won 89 of 137 games, with four draws.
His son Sam Irvine was part of the 1985 team, and said it was appropriate the seat looked out over the field where his father first ran out as a schoolboy in 1945.
"In some ways, he's never really left and it's fitting there is a seat dedicated to him."
Mr Irvine's grandson Otis helped Sam Irvine and Mr McLean lift the New Zealand flag to unveil the new seat.
Mr Irvine attended Collegiate as a student from 1945-49, becoming Prefect and making the First XV, later representing Wanganui in 27 games. He returned for three tenures teaching mathematics in 1958-77, 1981-90, and 1996.
Before coaching the First XV he built up the rowing programme in the early 1960s, with the WCS eight winning the Maadi Cup back-to-back in 1963-64.