Circa 1189 de Lacy is appointed Viceroy of Ireland, a position previously held by his father, Hugh de Lacy, Lord of Meath. He is replaced in 1190 by Guillaume le Petil. He is later reappointed to serve as viceroy from 1205 to 1210.
In 1199, King John authorises de Lacy to wage war on John de Courcy, who has conquered much of Ulster without help or permission from the King. Hugh captures de Courcy in 1204. An account of the capture appears in the Book of Howth.
After King John creates him Earl of Ulster in 1205, he makes what was de Courcy’s territory in Ulster the Earldom of Ulster. He grants Drogheda its charter and continues the conquest of the northeastern over-kingdom of Ulaid, building on de Courcy’s success, with the earldom spanning across the modern counties of Antrim and Down and parts of Londonderry.
In 1207, war breaks out between the Earl of Ulster and the justiciar. This brings King John to Ireland, where he expels the earl’s brother, Walter de Lacy, from Meath, and compels the earl himself to flee to Scotland.
For several years Ulster takes part in the wars in France, and de Lacy does not return to Ireland until 1221, when he allies himself with the O’Neills against the English. In 1226, his lands in Ulster are handed over to his brother Walter, but they are restored to him in the following year, after which date he appears to loyally serve the king, being more than once summoned to England to give advice about Irish affairs.
De Lacy purportedly separates from his first wife and lives in adultery. He has legitimate and natural children. In 1226, his daughter by his first wife marries Alan, Lord of Galloway. He marries his second wife, Emmeline de Riddlesford, the daughter of Walter de Riddlesford around 1242. Hugh de Lacy dies shortly thereafter in 1242 or 1243. Emmeline’s second marriage takes place around 1243 with Stephen Longespee, grandson of Henry II of England, by whom she has two daughters, Ela Longespee, Lady of Ashby, and Emmeline Longespee, Lady of Offaly.
Left with no surviving legitimate children, the earldom of Ulster reverts to the crown upon de Lacy’s death.