"of Scotland," 1590s, contraction of Scottish. Disdained by the Scottish because of the many insulting and pejorative formations made from it by the English (e.g. Scotch greys "lice;" Scotch attorney, a Jamaica term from 1864 for strangler vines).
Scotch-Irish is from 1744 (adj.); 1789 (n.); more properly Scots-Irish (1966), from Scots (mid-14c.), the older adjective, which is from Scottis, the northern variant of Scottish. Scots (adj.) was used in Scottish until 18c., then Scotch became vernacular, but in mid-19c. there was a reaction against it. Scotch Tape was said to be so called because at first it had adhesive only on the edges (to make it easier to remove as a masking tape in car paint jobs), which was interpreted as a sign of cheapness on the part of the manufacturers.