It depends on the whiskey. Most whiskies are made from fermented barley, but some like bourbon whiskey are made from corn, and rye whiskey is made from rye.
Some would say any way that you can get it. One very popular approach is the combination called Irish coffee, however the best of them should, like the great Scotch single malts be consumed straight...
Cant answer exactly but i will tell you that before the American Prohibition of alcahol Irish whiskey wasconsidered to be by far the finest. By the end of prohibition scotch had leaped in popularity...