The Coumshingaun Lough hike is virtually doable all year long. Realistically, you can really have a nice and fun adventure between the month of May and the month of September. The Irish weather is totally unpredictable, so you may end up getting soaked in June and having an amazing and sunny day in October. In wintertime, snow is not rare and so it's the ice; giving the location of the lake and the cliffs, it's not really recommended to venture in winter.