There is no piece of the dragon armor in Iona's dungeon. I believe you are thinking of the sword from the holy weapons quest.
Click here for info on the dragon armor. Since you missed the dragon shield in Stormfist castle, you can sneak back in by tossing a teleporter stone inside past the gate, or, when you try to enter and the guards stop you, click again immediately after and they won't respond fast enough to stop you.
Click here for info on the holy items.
You can still get back into Iona's dungeon - go back down to the graveyard and enter through the grave where you escaped.