I wonder what the first three "degrees of taboo" are

Let's see if we can figure it out!

The Irish system has degree n for ((n-2)/2)th cousin (for even values of n) and ((n-3)/2)th cousin once removed (for odd values of n).

The English system has nth cousin for latest common ancestor of (n+1) generations ago.

So, Irish *col beirt (2) would be zeroth cousin, which would mean parents as common ancestors, therefore it would be your siblings.
*Col triúr (3) would be zeroth cousin once removed, i.e., your siblings children, namely nephews/nieces.
*Col duine (1) is a slightly tricky one - your (minus one)th cousin once removed, which works out as your own children.