What effect did the arrival of the Rhoynar (with their far superior metallurgical/armouring skills) have on the development of plate armour in southern Westeros? Do you have a rough head canon timeline for armour development in Westeros from say 200 years before 10k Ships, up until 300AC? Also, are you of the opinion that without the advent of firearms, Westeros will not develop their plate armour to the RL pinnacle of 15th/16th century European armourers?

We don’t have a good deal of information about this. Here’s what we know:

“The fact that the Andals forged iron has been taken by some as proof that the Seven guided them—that the Smith himself taught them this art—and so do the holy texts teach. But the Rhoynar were already an advanced civilization at this time, and they too knew of iron, so it takes only the study of a map to realize that the earliest Andals must have had contact with the Rhoynar. The Darkwash and the Noyne lay directly in the path of the Andals’ migration, and there are remnants of Rhoynish outposts in Andalos, according to the Norvoshi historian Doro Golathis. And it would not be the first time that men learned of the working of iron from the Rhoynar; it is said that the Valyrians learned the art from them as well, although the Valyrians eventually surpassed them.”

“…The Rhoynar brought considerable wealth with them; their artisans, metalworkers, and stonemasons brought skills far in advance of those achieved by their Westerosi counterparts, and their armorers were soon producing swords and spears and suits of scale and plate no Westerosi smith could hope to match.“

So we know the Rhoynar had really well-developed iron and steel-working and had had it for a long time. And we know that they were making scale and plate which was much better than the Andals could make (who I guess were using chain?). Which is interesting, because whenever we see Rhoynar armor before Nymeria or indeed in Dorne, it’s all scale and not plate.

As for a timeline of technology, it’s a bit confusing. We know the First Men only had bronze, we know the Andals learned iron (as opposed to steel) from the Rhoynar, but Andal invaders are described as having “steel swords and iron ringmail,” which suggests an intermediate stage of development). That quote suggests that plate armor only came to Westeros 700 years ago, but we also hear that the Marcher castles were built to “guard the borders of the stormlands against…the steel-clad minions of the Kings of the Reach from the west.” 

But technology is uneven, so transfers could come from many places. For example, we know that the Valyrian and Ghiscari Empires had steel plate (since Xanda Qo of the Summer Isle built bows to penetrate plate armor during the Slavers’ Wars) more than 5,000 years ago.