Umhaill or Umhall (anglicized as Owill or Owel) was a Gaelic territory around Clew Bay in the west of what is now County Mayo, Ireland, comprising the baronies of Burrishoole (Lower Owel) and Murrisk (Upper Owel). By the 12th century, its ruling dynasty were known as the UàMáille (O'Malleys). Originally an autonomous part of the kingdom of Connacht, it later became one of the vassal territories of the Mac William ÃÂochtar. Umhaill's last and most famous ruler was Grace O'Malley (Gráinne NàMháille), nicknamed "the pirate queen". In 1576, during the Tudor conquest of Ireland, she agreed to the surrender and regrant policy, accepting English inheritance law in return for official title deeds to her lands. On her death the lands were inherited by her son Tibbot "na Long". Umhaill had a strong seafaring culture. Important sites associated with it include Carrickkildavnet Castle, Carrickahowley Castle, Granuaile's Castle and Clare Island Abbey.
Knox says of Murrisk "With Burrishoole it forms the kingdom of Aicill and Umall, which comes into history at the battle of Moy Lena. Aicill seems to be a descriptive term applied to mountainous country. Umall means low, and applies in this sense to the country lying east of Clew Bay, as Aicill applies to the parts lying north and south of the bay. The title may be translated as King of Highland and Lowland. Aicill survives in Achill Island and Curraun peninsula. The term was applied to the country between Clew Bay and the Killeries in the thirteenth century. Though at all times an independent kingdom acknowledging supremacy of only the King of Connacht, it was too small to play an independent part, and therefore is rarely mentioned in the annals." (p. 303).
The Gaelic culture and Irish language continued on in the area longer than most other parts of Ireland. Today, Acaill and An Corrán are part of the Mayo Gaeltacht.
Its earliest rulers were the semi-historical Tuath mhac nUmhoir. The Umaill, its early historical rulers, were renamed the UàBriúin Umaill to claim a fictitious relationship with the UàBriúin. By the 12th century the ruling family adopted the surname àMáille, and were reckoned with the àDubhda, àFlaithbheartaigh and Mac Conraoi as supreme seafaring clans of Connacht.