In September, the weather in Carrick-on-Shannon is typically warm. Average daily high temperature in September is about 16 degrees above zero. On average, Carrick-on-Shannon has 15 rainy, 8 cloudy, 5 partly cloudy, and 2 sunny days in September. The rains are usually heavy. September as well as January, February, March, and April is considered the most uncomfortable time to visit Carrick-on-Shannon.