O'Sullivan family in the context of Eóganacht Chaisil

The Kingdom of Desmond (Irish: Deasmhumhain, meaning 'South Munster') was a historic kingdom in southwestern Ireland. It was founded in 1118 by Tadhg Mac Cárthaigh, King of Munster when the Treaty of Glanmire formally divided the Kingdom of Munster into Desmond and Thomond (Tuadh-Mhumhain, "North Munster"). It comprised all of what is now County Cork and most of County Kerry. Desmond was ruled by the Mac Cárthaigh (MacCarthy) dynasty. Other clans within the kingdom included the O'Sullivans and O'Donovans. Following the Norman invasion of Ireland in the late 12th century, the eastern half of Desmond was conquered by the Anglo-Normans and became the Earldom of Desmond, ruled by the Fitzmaurices and FitzGeralds—the famous Irish family known as the Geraldines. The king of Desmond, Diarmaid Mac Cárthaigh submitted to Henry II of England, but the western half of Desmond lived on as a semi-independent Gaelic kingdom. It was often at war with the Anglo-Normans. Fínghin Mac Carthaigh's victory over the Anglo-Normans at the Battle of Callann (1261) helped preserve Desmond's independence. The kings of Desmond founded sites such as Blarney Castle, Ballycarbery Castle, Muckross Abbey and Kilcrea Friary. Following the Nine Years' War of the 1590s, Desmond became part of the Kingdom of Ireland.