Re: Irwin 28 head help
Not knowing when your boat was built or what model it is, I'm guessing that the holding tank was underneath the port settee. You say the pumpout piping is still there, so the holding tank had to been in the same spot.
Just start looking around. There are only so many places where the tank can be located, it is probably obvious. For a basic sanitation system, all you really need is a new, marine head (toilet) and you probably already have the water intake thru hull to supply water to it. As I said, the holding tank is probably just behind the bulkhead under the settee, so you need a hose running from the head to the tank and another hose/pipe running from the tank to the deck pump out. You will also need a vent hose for the tank.