Lughnasadh is mentioned in some of the earliest Irish literature and has pagan origins. The festival itself is named after the god Lugh. It involved great gatherings that included religious ceremonies, ritual athletic contests (most notably the Tailteann Games), feasting, matchmaking and trading. There were also visits to holy wells. According to folklorist Máire MacNeill, evidence shows that the religious rites included an offering of the ‘first fruits’, a feast of the new food and of bilberries, the sacrifice of a bull and a ritual dance-play in which Lugh seizes the harvest for mankind and defeats the powers of blight. Much of the activities would have taken place on top of hills and mountains. (from Wikipedia)

Originally it was held on 1 August, or about halfway between the summer solstice and autumn equinox. However, over time the celebrations shifted to the Sundays nearest this date

This is a celebration of the early harvest. Many traditions and myths surround this season. Climbing to the top of a hill or mountain, giving the first fruits or harvest as offerings of thanks, mock battles or games that test strength and courage.