Irwin, who won the ACE Group Classic last month in Florida for his record 33rd senior title, had an 11-under 131 total.
'A good start yesterday helped my momentum,' Irwin said. 'If we could continue playing today I feel the momentum would continue.'
The three-time U.S. Open champion made eight birdies and a single bogey but credited a par-saving putt on the seventh with his good round.
'The seventh hole turned things around,' said Irwin, using a new set of irons for the event. 'I made a miracle par. My tee shot was in the rough, my second shot was in the rough but I hit my new sand wedge within 3 feet and made the putt to save par.'
Larry Nelson matched Irwin's 64 for a three-way tie for second with Gil Morgan (66) and first-round leader Allen Doyle (68).
Dan Pooley, four strokes back at 135, had the best shot of the day, holing a 50-yard eagle chip on the 15th hole. He had the eagle, six birdies and two bogeys in a 65.
Thirty-two players have broken par over the par-71 Newport Beach Country Club course, but Tom Watson's string of 12 straight rounds at par or better ended Saturday with a 1-over 72. It was his first round over par this year. He was nine strokes back at 140.