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To my knowledge the only connection between Ciarrai and O Ciardha is that both names incorporate the word CIAR, which, as you point out, means "dark." Ciarrai was the name of the population group which gave the County Kerry its name. The ancient Ciarrai, descendants of an early mediaeval Ciar, are now represented in the main by the numerous O'Connors of Munster. Ciarrai was never a surname.
The O Ciardha clan, or clans, don't belong to the County Kerry.
The surname Carey is English in origin and was adopted by O Ciardhas as the English language replaced the Irish. The surname Kerry, I believe, is English too, and not connected to the Irish place-name. I can only guess at its meaning, perhaps an alternative form of Carey or Kerrich.
Senator John Kerry's origins lie in Central Europe. His family adopted their present surname after settling in the US.