Sil was born into Tuatha de Bhriain as the son of Dairmuid Mac Domhnaill and his wife Banva. He was fostered to Echu the druid.

The life of a druid did not interest Sil and as a result he found his rump under the hazel rod more often than the other accolytes. As he grew older, he planned to run away, though it would bring dishonor upon his name. One day, Sil overheard of a raiding expedition. This was his chance. He stole away from the druid and stole his torc as well. He hid in the rocks by the beach, waiting for the rovers to depart. As the men were pushing out to sea, Sil ran and jumped aboard. The warriors asked him his business and Sil told of his plight. The men were impressed with Sil. They thought it a waste for anyone with a warrior's heart to spend his youth as a druid's accolyte.

Though Sil was young, Crimmal, the captain, remembered the boy deeds of Cuchulainn and regarded Sil as a bringer of luck and fortune. The band harried the coast of Britain, winning many rare and exotic treasures. Sil attended to many warrior tasks. He mended shields, sharpened swords, and hewed javelin poles. The warriors rewared his spirit by teaching him their favorite fighting tricks and how to fight with deadly rage. At fourteen, Sill had killed three men and had a share in the spoils. On returning home, Dairmuid was told of his son's deeds and Sil was received with honor. He remained in fosterage for his last three years with Crimmal the captain. Since his coming to manhood, he has claimed the champions' portion more than once.