Lag, Ireland offers 2 beaches perfect for sunbathing, surfing, swimming, and more. Lag, Ulster, Ireland, is known for the historic Lag Castle and the scenic Lag River. The best time to travel to Lag, Ireland, is during the summer months of June to August, when the average air temperature is around 15-20°C (59-68°F) and the water temperature is approximately 14-16°C (57-61°F). Sandee provides detailed maps, photos, and reviews from fellow beachgoers to help you Choose Your Beach™ and plan your perfect beach vacation!