The first bearer of the name was Ceallach, son of Finnachta, a chieftain of the people of Hy Many c.AD 874. Ceallach means war or contention. For centuries, they were one of the most powerful families in Connacht, mainly in counties Galway and Roscommon. Kilconnell Abbey, near Aughrim in Co Galway, was founded by the O'Kellys in the early-15th century and its ruins are still extensive. There is also an ancient O'Kelly castle on the shores of Lough Conn in Co Mayo.

Ned Kelly, the notorious Australian outlaw who died in 1880, had Irish connections: his father, Red Kelly, came from Tipperary town. Gene Kelly, born in 1912 in Philadelphia, was Irish on both sides of his family; he became renowned in Hollywood as a singer and dancer. Grace Kelly (1929-1982), also from Philadelphia, had outstanding success as a Hollywood film actress before marrying Prince Rainier of Monaco and becoming Princess Grace of Monaco. Her grandfather was an Irish immigrant; she traced her roots to near Newport, Co Mayo.