It could be said on a micro scale, that Kayce and the lord of FairIsle could be descendants of iron born lords that were given/taken land and then became a part of the westerlands polity.

Well, the Farmans are First Men. 

The Kennings of Kayce are a complicated story. The founder of their House, Herrock Kenning, was known in his time as Herrock the Whoreson, and wasn’t given land in the Westerlands. Rather, he engineered an uprising against the Ironborn garrison of Kayce, with the help of the prostitutes of the town who opened the postern gate to let his men inside the walls. And then he held the town against three separate Ironborn kings who he personally killed.

To me, this suggests the following scenario. Herrock was born to one of the sex workers of Kayce, sired by a Kenning of Harlaw. However, I think his paternity was always challenged or in doubt – hence the derogatory nick-name – and that, seeing that he could never rise within Ironborn society because of his birth, he sided with his mother’s people and that the women who let him in did so because they knew him through his mother. 

And given what we know about the Ironborn-descended people of the North and other areas on the Sunset Sea, I think this goes further than assimilation. Remember, the Normans called themselves Normans, had distinctively Norman hairstyles and clothing, spoke a distinctively Norman French. But the Kennings of Kayce or the men of Cape Kraken don’t think of themselves as half-Ironborn; they hate the Ironborn with a passion and reject that identity and association completely.

Why is this? Well, part of it comes from the Ironborn tradition of taking salt wives and otherwise sexually assaulting any non-Ironborn they want – not exactly likely to engender mothers to raise their children to love their fathers. Another part of it comes from Ironborn ideology – greenlanders are essentially thralls waiting to happen, and it’s the birthright of any Ironborn to take what they like whether or not the greenlander has paid tribute. I doubt highly that half-castes on the mainland are given any exemption from that sort of treatment.