The name could be of Scottish, English or Irish origin and is indeed found in all three countries. If English, it would come from the Middle English first (or given) name Irwyn, Erwyn, Everwyn, which in turn is from Old English 'Eoforwine', meaning 'Wild boar' + 'wine' (friend).
Another origin is the Anglicized form of the Irish, O'hEireamhoin, meaning descendant of Eireamhoin, a personal name of uncertain origin. Eireamhoin was one of the mythological sons of Milesus, the legendary leader of the Gaelic invaders of Ireland. All three of the sons' names, it has been suggested, are variations of the name for Ireland itself ...which ultimately comes from an Indo-European root meaning 'fertile' land.
If it is of Scottish origin, the name likely comes form the river in Ayleshire, Scotland. There is some confusion as to whether it means West River, Brown River or White River. However, it should be noted that Scotland was colonized by the Irish and and the Scottish also later settled in Ireland while the English settled in both. Therefore, it is tricky to determine your specific family's name origin unless you have more historical knowledge of your family's specific geographic origins.