The English cousin system takes a little getting used to, but is quite straightforward: first cousins share a pair of grandparents, second cousins share great-grandparents and so on. Then, your first cousin's child is your first cousin once removed, his/her grandchild is first cousin twice removed, etc., the number of removals being the number of generations that separate you. The combination of the two numbers pinpoints a location on the family tree like a knight's move: across and then down.


Irish is quite different:
col ceathrar (4) -> first cousin
col cúigir (5) -> first cousin once removed
col seisir (6) -> second cousin
col seachtair (7) -> second cousin once removed
col ochtair (8) -> third cousin

Now, "col" means "taboo", "incest", "prohibition", "repugnance", "impediment to marriage". And "ceathrar", "cúigear", "seisear", "seachtar" and "ochtar" mean 4, 5, 6, 7 and 8 (in the form used for counting people).

So literally, in Irish, your first cousin is your "incest to the fourth degree", your second cousin "to the sixth degree" and so on. (Needless to say, we don't think of this when we use the words!)

It makes a kind of logical sense, except for being so very explicit and legalistic, and for the fact that cousin marriage is not really a taboo in Ireland, even for first cousins, let alone third! I wonder if the pattern continues for fourth and subsequent cousins?