Gaelic variations:
Ceallaigh, Ceallach, Ó Ceallaigh, Ó Caollaidhe, Ó Caollaighe, Ó Cadhla, Mac Caochlaoich: “war or contention”.
Anglicised variations:
Kelly, Kelley, O’Kelly, Kealy, Keely, Keily, Kehelly, Kehilly, Coakley.
Kelly is the second most common surname in Ireland and originates from at least seven different and unrelated ancient clans or septs. These include O’Kelly septs from Meath, Derry, Antrim, Laois, Sligo, Wicklow, Kilkenny, Tipperary, Galway and Roscommon, and the McKelly sept from East Connaught.
One of the major septs is the O’Kellys of Ui Maine, or Hy Many, a territory in East Galway and South Roscommon. O’Kelly derives from the Gaelic O Ceallaigh, meaning “descended from Ceallach.” Ceallach was the son of Finnachta, a chief of the Hy Many people around A.D. 874.
Here’s a list of various clan websites:
Kelly Clan Association – Muintir Uí Cheallaigh
Kelly Clan Association – Kelly Places and People
The Kelly Clan, County Armagh, Northern Ireland.
Kelly may also mean ‘bright-haired’ or ‘troublesome’ – you choose! 
Just beginning to trace my family tree. FatherKelly, Mother O’Shea. I live in USA
Hi Mary,
Thanks for joining and good luck with your research – always a challenge but very rewarding.
If your family were still in Ireland in 1911 keep an eye on http://www.census.nationalarchives.ie/
(It does'nt include Clare yet but hopefully should do later this year).
All the best,
Mick