From Wikipedia's "Cusack" article:
Cusack is an Irish family name of Norman origin, originally from Cussac in Guienne (Aquitaine), France. The surname died out in England, but is common in Ireland, where it was imported at the time of the Norman invasion of Ireland in the 12th century.
While not particularly common, the name Cusack is historically associated with a number of variant forms, such as de Cussac, de Ciusak, de Cíosóg, de Ci'omhso'g, MacIosog, and Mac Isog. Irish emigrants to America were given alternative spellings by immigration officers, based on their pronunciation. Hence, Kuzak, Kuzack, Cusick, and Cussack are modern variants in America.
The first mention of de Cusack's arrival in Ireland appears in The History of England by Paul Rapin de Thoyras. This records that a 'Seigneur de Cusac' had accompanied William, Duke of Normandy in his Conquest of England in 1066.
It was in 1172 that Geoffrey de Cusack arrived in Ireland during the reign of King John, possibly at the invitation of his relative, Adam de Feypo. The family served as Lairds of Killeen (ranked below a Baron and requiring Knight Scutage i.e. the supply of knights for 40 days service per year to Hugh de Lacy, Lord of Meath, when requested).
Cusack family connections start from when Geoffrey de Cusack arrived in Ireland from France in 1172, built his castle at Killeen in 1181, married Matilda Le Petit, daughter of Adam Le Petit Lord of Dunboyne, sired three males and died early in the 13th century. Included are references to the estates acquired by some of his descendants as well as descriptions of the family Crest and Motto also the Memorial Stones and the fine Epitaph to Sir Thomas Cusack and the last of the Cusacks at Killeen - Lady Joan de Cusack.
The de Cusack Royal French connection can be studied in the Generlogiques et Historique de la Noblesse de France and also in Genealogie de la Maison de Cusack. The particules - "de" or "de la" - are badges of nobility and are only recognised by letters patent from a king of France.
The de Cusack family originally held land and influence over present day Cussac-Lamarque in the Medoc region of France, north of Bordeaux in what was then Aquitaine.
Geoffrey de Cusack came to Ireland in 1172. This was soon after the first Normans landed in Ireland in 1169.
Geoffrey was granted the manor of Killeen, 3,346 acres (13.54 km2), by Adam de Feypo, a relative who obtained his lands by charter and like Adam he was subject to Hugh de Lacy, Lord of Meath.
Geoffrey married Matilda le Petit, sister of William le Petit who held a Barony at Mullingar and a castle at Donboyne sometime before 1181 and bore him at least two sons, Adam and William (English Public Records) however Irish Languish Pedigrees believe his eldest son was Geoffrey II.
Geoffrey de Cusack died between 1210 and 1218 he is the first of long lines of Cusacks in Ireland. Both the medieval and modern Cusack lines and genealogy of Geoffrey's offspring have been traced in great detail by Lt. Colonel Hubert Gallwey.
In 1399 the manors and estates of the Lordship of Killeen passed by the marriage of Lady Joan de Cusack to Christopher Plunkett of Rathragen.