Kelly comes from the Irish Ó Ceallaigh, based on the popular personal name Ceallach, which may mean either "bright-haired" or "troublesome". The popularity of the name meant that it was incorporated into permanent surnames in between seven and ten different places, including Co. Meath, north Wicklow, the Antrim/Derry area, Co. Sligo, Galway/Roscommon, north Down and Co. Laois.
The most prominent of these families are the O'Kellys of Uí Maine, or Hy Many, an ancient territory taking in east Galway and south Roscommon, also known simply as "O'Kelly's Country". Their pedigree takes them back to Maine Mor, first chief of the area bearing his name, who lived in the fifth century. His descendant, Ceallach, (died c.874) was the twelfth Chief, and it is from him that the surname derives. His great-great-grandson Tadhg Mór, who died at the battle of Clontarf in 1014, was the first to use the name in true hereditary fashion.
Despite the loss of most of their possessions in the catastrophic wars of the seventeenth century, a loss shared with most of the rest of the Gaelic aristocracy, the succession to the position of head of the sept has continued unbroken down to the present incumbent, Walter Lionel O'Kelly of Gallagh and Tycooly, Count of the Holy Roman Empire, known as "The O'Kelly", and recognised as such by the Chief Herald of Ireland
Today, Kelly and O'Kelly are almost as numerous in Ireland as Murphy, and are to be found throughout Ireland. Individuals of the name have been prominent in all spheres of Irish life. The best-known modern Irish sculptor was Oisin Kelly (1915-1981); Charles E. Kelly (1902-1981) was one of the founders of Dublin Opinion, the most famous satirical magazine to appear in Ireland; James O'Kelly (1845-1916) had a remarkable career as a war correspondent and Member of Parliament .