The most pleasant months of the year for Irwin are May, September and June. In Irwin, there are 4 comfortable months with high temperatures in the range of 70-85°. August is the hottest month for Irwin with an average high temperature of 89.3°, which ranks it as warmer than most places in Missouri. January is the snowiest month in Irwin with 4.0 inches of snow, and 4 months of the year have significant snowfall. July is the most humid month, though humidity is low for most of the year.