Fergus mac Léti

Fergus mac Léti (also mac Léte, mac Léide, mac Leda) was, according to Irish legend and traditional history, a king of Ulster. His chronology is uncertain – some sources place him as a contemporary of High King Conn of the Hundred Battles, while others associate him with Lugaid Luaigne, Congal Cláiringnech, and Fachtna Fáthach.

The Caithréim Conghail Cláiringhnigh tells how Fergus ruled southern Ulster while Congal Cláiringnech ruled the north. When the Ulaid objected to having two kings, High King Lugaid chose Fergus as sole ruler and gave him his daughter Findabair in marriage. Congal, refusing to accept this judgment, eventually overthrew Lugaid, installed himself as High King, and deposed Fergus. Later, during Fachtna Fáthach's reign, Fergus was restored as king of Ulster.

In his most famous tale, Fergus encounters lúchorpáin ("little bodies") – considered the earliest reference to leprechauns in Irish literature. After capturing these water-sprites, they grant him three wishes, including the ability to breathe underwater. However, this power wouldn't work in Loch Rudraige. When he attempts to swim there anyway, he meets a sea-monster called Muirdris that permanently disfigures his face in terror. His subjects hide this from him for seven years until a serving girl reveals the truth. Fergus returns to the loch, kills the monster after a bloody two-day battle, but dies from exhaustion.

Some scholars identify him as a double of Fergus mac Róich due to similarities in their stories, though both characters appear together as enemies in the Caithréim Conghail Cláiringhnigh. His legend later influenced popular culture, including Disney's "Darby O'Gill and the Little People."