Oracle of Seasons
After Link obtains both the Blue Ore and the Red Ore, he can bring them to the Great Furnace to obtain the Hard Ore. Link can then take the Hard Ore, along with the Wooden Shield, to the Subrosian Smithy, where it is forged into the Iron Shield.
Oracle of Ages
When Link first arrives at Crescent Island, the Tokay steal all of Link's items, including his Wooden Shield, if Link has acquired one. Link never finds this particular Tokay the first time traveling on the island and instead, can purchase the Wooden Shield from a Business Scrub located on the island. It isn't until much later in the quest where he finds the Tokay at the south end of Crescent Island, who says that he has been polishing up the shield, turning it into the Iron Shield.
If Link has already acquired the Iron Shield using the Library Secret, then the shield found on Crescent Island will be the Mirror Shield. If Link acquires the Iron Shield on Crescent Island first, then he will get the Mirror Shield at the Eyeglass Library, using the Library Secret.
The Iron Shield is the second shield Link can purchase in Skyward Sword. This shield is much more durable than the other shields Link can purchase. It has resistance to fire-based attacks, but conducts electricity. This shield can be obtained before Link sets out for Eldin Volcano at a cost of 100 Rupees. Link can use Treasures to upgrade to the Reinforced Shield and the Fortified Shield.