Donnelly is a city in rural Valley County, Idaho, United States. The population was 152 at the 2010 census.Donnelly is in the Long Valley of Valley County, on the northeast shore of Lake Cascade, between the two major cities in the county. McCall is 13 miles north, and Cascade is 16 miles to the south, via Highway 55. Tamarack Resort is across the reservoir to the southwest.