Lough Derg is at its best from May to September, when the days are long, the sailing season is in full swing, and boat trips to Inis Cealtra run regularly from Mountshannon. July and August are the busiest months — marinas and harbours fill up with hire cruisers and private yachts, and the lakeside pubs do brisk trade.
June is often the finest month: the hedgerows are in full flower, the days are long without the August crowds, and the weather is as reliable as Irish weather gets. May can be cold but is excellent for fishing.
Autumn brings quieter roads and a melancholy beauty to the lake — morning mist on the water, the Clare hills turning amber — though boat trips become less frequent after October. Winter is genuinely quiet; most of the seasonal activity shuts down, and the lake reverts to its year-round residents: anglers, local sailors, and the odd hardy walker.
The Lough Derg Yacht Club races and regattas peak in July and August, and Dromineer Jazz and Blues Festival (usually late September) is the most popular cultural event on the lake's calendar.