They was a magnificent feast to celebrate the newly engaged couple. People travelled from all over the country to toast the couple. On the night of feast, before Gráinne entered the great hall, she looked in. She saw the ageing Fionn at the centre of the top table, and to his right was Diarmuid, the black-haired boy from her youth. On seeing Diarmuid, Gráinne knew that she could no longer marry Fionn.
As Gráinne sat to the feast, she passed round a cup of wine, filled with a sleeping potion, to all except Diarmuid. When all fell asleep, Gráinne told Diarmuid about when she fell in love with him and how she could no longer marry Fionn. She told him she wanted to run away with him so they could be together. But Diarmuid was loyal to Fionn and wouldn’t betray Fionn. Gráinne put a spell on Diarmuid so he would fall in love with her, which he did, and they ran away together.
Shortly after running away together, they met Aengus Óg, the Celtic god of love and Diarmuid’s foster father, and he approved of the match so much he decided to help them. He told them they would be unable to sleep in a cave with one opening or a house with one door. They could never eat where they cooked, or sleep where they ate. They would have to keep moving if they were to stay ahead of Fionn and the Fianna.