The main reason for this variety is the 'Anglicisation' of many Irish place and fore/surnames as a consequence of the colonisation of Ireland (particularly from the 16th century onward) and the subsequent imposition of the English language.
As many Irish migrated during the 19th and 20th centuries (especially in the period 1845–1851 to escape the 'Great Famine') the surname can be found in all major English-speaking countries of the world today.
According to O'Hart's "Irish Pedigrees", Burke's "General Armory," and Fairbairn's "Book of Crests" these include: Most commonly: ARMS/CREST: "A mount in base vert an oak tree proper a bordure of the second.
A dexter cubit arm in armour proper garnished or and gules holding a flaming sword azure pommel and hilt or."
MOTTO: "Certavi et vici" ('I have fought and conquered') Alternatively (Burke): Flanagan (Drumdoe, co. Roscommon).