The Hull and Deck:
Starting with the deck, it's wet. Really. I don't care what you've been told, it's wet. If the port lights are original, they've been leaking for years and the areas around them and where ever the water has gotten to, have dry rotted into junk. The chain plates are surrounded with wet core as well. As for the rest of the deck, if you are up north where the water in the core can freeze and swell, the deck probably has lumpy spots in it too. Every place where the fiberglass has been drilled into, including the stanchions, leaks. This sounds bad, but you can see the work and expected it anyway. All this is a given, in a 30 year old boat. There are dozens of threads on replacing wet core, I'm not going to discuss it here. Maine Sail has excellent commentary on the process as does Casy in his books on sailboat repair. Here is a picture on 1/2 of the rotted wood removed from my port Irwin settee.
What few people will tell you, but I will suggest, is that there are a great many boats sailing around with wet core. Its impossible for me to say the condition of the core in your boat, to say how bad it is, but unless the deck feels like walking across a soft mattress, I wouldn't go into catastrophic fits of despair. If the deck is reasonably solid and a local repair around the chain plate can be made solid and structurally sound, then why tear up the whole deck ? You are the only one who can evaluate your specific boat.
The ash blocks which are the bases for the Irwin stanchions, are most probably rotted out. These are glassed into the hull and you will have to figure out how to fix them. I dug mine out with screwdrivers bent into hooks and used compressed air to blow out the debris. Then filled the places where the ash used to be with fiberglass filled bondo. Everyone will figure out their own solution. I like mine well enough.
The toe rail will leak. This is a special repair. Easy to pull off and re-bed with butyl tape, you can wreck your boat on re-assembly. The screws that hold the toe rail on also are part of the hull/deck structure. Do not refasten them with power tools. Hand screw in the screws, so as not to destroy their fastening ability. These holes are easy to strip out, and the screws, if stripped, will quickly work loose and probably leak again. Just my opinion.
Tabbing: The Irwin was built when the use of the interior as part of the hull stiffening structure, was still new. It seems to me there was a confusing assortment of techniques used to attach the hull to the interior structures. It ended badly. The screws in the sole are useless. The tabbing through out the boat is probably torn away from the hull as well. Check in the forward compartments under the V birth, in the head cabinet, under the quarter birth, by the fuel tank under the star board settee, and the main bulkhead, of course. When you re-attach, remember not to make the repair too robust so to create hard spots which will crack gelcoat on the hull exterior.
here's completed repairs to the above photo: