Having spent a great deal of time in both Countries, I can vouch that both scotch and irish whiskey can be blended. However both can be singles also. Depends on where you get it.
What lands in the US from Ireland is typically blended. Whereas a lot of scotch comes in as single malts. Earlier someone posted that the easiest way to tell the difference is by the peaty/smokey taste. This is completely false. They both have those that have been aged in peat casts.
It really difficult to tell the difference. I personally like Irish Whiskey around St. Pats day and single malts around Robbie Burns Birthday.