Historically, handheld shields were never made wholly of metal or covered with much metal plating on the outside. That would make them too heavy and diminish their mobility. You often see shields with a metal bosse or with a metal hoop or frame to protect their edges, but they were predominantly made of wood and hardier cloth. About the only mostly-metal historical shields I can think of were the small types, for duelling or personal self-defence, such as the tiny buckler or the slightly larger targe.
I suppose partially iron-shielded or steel-shielded mantlets and pavises could work. However, these portable static shields need to be made of an extra-hard wood as the base, then have some additional padding attached on the front (maybe even just gambeson-style wool-in-thick-cloth padding), and on top of that, a thinner bit of hard-wooden shielding, with a thinner layer of iron or steel shielding attached. This setup could potentially thwart even 19th century guns, but just narrowly. Some of the better rifles later in the century would probably drill through even such multiple-layer portable shields.
Even if the materials are light, the mantlets or pavises will still be very heavy due to having the outside armoured coat, not to mention all the other (somewhat lighter) layers of armouring. Industrial era firearms saw a gradual, but firm increase in accuracy and muzzle velocities, and that is part of why we stopped using armour and shields made of traditional materials to counter cartridges. Simple metal armour isn't entirely ineffective against modern firearms, but it leaves a lot to be desired in terms of effectiveness. Part of why materials like kevlar were developed - strong and elastic enough to withstand a small amount of shots, but light enough to allow for a lot of active movement during combat.
Mail armour (i.e. the ring one, for mail shirts, etc.) is a very old concept, invented already during the late Iron Age. It survived in use since antiquity until at least the 18th century (in parts of the world). Early attempts at mail armour from the Iron Age were fairly unrefined, and many were made of simple iron, rather than steel. I suppose that would be your best bet for "iron armour". Beware though, mail armour didn't really take off before blacksmiths learned to make proper steel during antiquity.
