I was just about to launch into an explanation with someone about how logical the whole 'cousin' thing in Irish is and suddenly found that I couldn't explain the 'ceathrar' part of 'col ceathrar' - first cousin. I certainly knew this before, but just cannot figure it out now. There are only two grandparents in common so that doesn't help me. I know that 'col seisear' is second cousin and I think that 'col cúigear' is the one in between ie first cousin, once removed and so on.

Can somebody explain the 'ceathrar' part of 'col ceathrar' please?

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I think it is because each cousin has two parents - making four - two of which are siblings - making each cousin one of a four-part relationship.

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Actually, the number is based on the number of links/persons one has to go through to get to the relative in question.
Thus, first cousins are col ceathrair/ceathar because you go:
(1) from cousin1 up to his/her parent (parent1), then
(2) from parent1 up to parent1's parent (the grandparent of cousin1), then
(3) from that grandparent down to parent1's sibling (call him/her parent2), and finally
(4) from parent2 down to his/her child, who is the other first cousin.

Editing to make it fully gender-neutral.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

That's a very helpful explanation and explains the col seisear etc also. I knew it was all very logical, but just couldn't remember how it worked.

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