Also, as I understand it, dark hair is common among Celts - dark hair and pale eyes, strikingly.
And here I heard that it was common for Celts to have fair hair and dark eyes. It probably just goes to show that such attempts at reconstructing what was an ancient Celt like (or any other elusive people) are very unreliable. Especially more so, because it's very debated who the Celts were: an ethnicity, a high caste with the same genetic/ethnic origin, a wide culture adopted by different people that didn't share the same origin, a mixture of all of the above, etc.?
More on topic, I always had a strong disliking of History as told from the perspective of people A invaded people B, therefor people B were all killed and nowadays we all descend from A. It seems to me that the current populations of most of Europe are genetically related to those that already were there thousands of years ago, and subsequent invasions and language changes only had limited impact in the genetic pool. It wasn't until modern times, that travel and communications improvement and industrial economic development allowed for mass scale immigration I think.
Btw, weren't there earlier genetic studies that very much related the present population of the British Isles with that of Northern Spain and Portugal, which is also traditionally considered as pre-roman Celts?
Nowadays both regions may seem very different, but Iberia is historically split in two greater geographic and cultural areas: the wet, mountainous and atlantic north, and the hotter, plainer and mediterranean south, and until the modern nation states, Iberia looks like a mini-Europe, with the North strongly linked with Northern Europe, and the South linked to the mediterranean.
