From Wikipedia

"The Ulster Cycle is peppered with references to Samhain. Many of the adventures and campaigns undertaken by the characters therein begin at the Samhain Night feast.

One such tale is Echtra Nerai ('The Adventure of Nera') concerning one Nera from Connacht who undergoes a test of bravery put forth by King Ailill. The prize is the king's own gold-hilted sword. The terms hold that a man must leave the warmth and safety of the hall and pass through the night to a gallows where two prisoners had been hung the day before, tie a twig around one man's ankle, and return. Others had been thwarted by the demons and spirits that harassed them as they attempted the task, quickly coming back to Ailill's hall in shame. Nera goes on to complete the task and eventually infiltrates the sídhe where he remains trapped until the end of the world. Taking etymology into consideration, it is interesting to note that the word for summer expressed in the Echtra Nerai is samraid.

The Cath Maige Tuireadh (Battle of Mag Tuired) takes place on Samhain. The deities Morrígan and Dagda meet and have sex before the battle against the Fomorians; in this way the Morrígan acts as a sovereignty figure and gives the victory to The Dagda's people, the Tuatha Dé Danann."