The most pleasant months of the year for Irwin are September, June and August. In Irwin, there are 2 comfortable months with high temperatures in the range of 70-85°. July is the hottest month for Irwin with an average high temperature of 87.3°, which ranks it as warmer than most places in South Dakota. March is the snowiest month in Irwin with 8.5 inches of snow, and 7 months of the year have significant snowfall. But there some days when the humidity becomes unpleasant, especially in July and August.