1970s boats typically had the fuel tank either in the bilge or below the cockpit sole. The fuel tank is probably on the centerline or very close to it. On an Irwin I would look beneath the cockpit for evidence of old supports. Look for wood cleats or remnants thereof or old tabbing that would have held the supports in place.
Another clue will be the fill hose. Some boats, Pearson 30s had the tank beneath the cockpit sole with the fill line in the cockpit sole. Look for a fitting or a patch in sole. The advantage to this location was a short fill hose and the ability to use a dipstick to measure fuel level.
Time for detective work in dark cramped spaces.