Q. Do you capitalize Scotch when it’s used singularly as a noun, or only in a proper name situation: Let’s have a scotch. I drink Scotch whisky and Irish coffee. Thank you!
A. According to CMOS, 16th ed., 8.60,
Personal, national, or geographical names, and words derived from such names, are often lowercased when used with a nonliteral meaning. For example, the cheese known as “gruyère” takes its name from a district in Switzerland but is not necessarily from there; “swiss cheese” (lowercase s) is a cheese that resembles Swiss emmentaler (which derives its name from the Emme River valley).
Among the examples included at 8.60 is “scotch whisky.” But in your example, you are right to capitalize “Scotch” in “Scotch whisky” because you are opposing “Scotch” to “Irish.”