A col is a degree of relationship. The degrees of relationship between you and your first cousin are:
- your parent;
- your parent’s parent (your grandparent);
- your parent’s parent’s child (your parent’s sibling);
- your parent’s parent’s child’s child (your first cousin).
Ceathrar is the word for “four” when referring to persons.
It should be clear that the numbers relate only to your blood relationship with the cousin - so your col ceathrar (4) would be a) you, your mother, her sister or brother, and their child, or b) you, your father, his sister or brother and their child. The extra spouses aren't counted in.
'Col ceathar' doesn't. That, as scilling noted, refers to the fourth degree of relatedness.
'Col', on the other hand, can do, but it's something of a difficult word to translate accurately. Its primary meaning is about how closely related you are to somebody, and thus secondarily to the incest taboo, but can be more generally to wickedness, aversion, violation, &c. It's not a euphemism though. I think 'taboo' is about as close to an accurate translation as you'd get, but even that's inaccurate.
Col on its own means an impediment, an objection.
col pósta is an impediment to marriage.
col-scaradh = divorce, literally "an impediment to separation".
Ó Dónaill gives ciorrú coil as "a violation of an impediment to marriage; incest". The more negative part of this phrase is ciorrú which means "a violation of" so it's the violation of the impediment, not the impediment itself, that is negative.
There is also a verb col. e.g. colaim means "I pick out and reject as worthless". Perhaps it is related to the English word "cull" as in the culling of deer or cattle.