I am not related to this surname but have looked at phone listings on-line that show the following:
Canada = 50; most are in Ontario, New Brunswick, British Columbia (ON and NB likely arrived 1820-1850 period)
USA = 156; most in MA, CT, NY, NJ, CA, FL, and assorted mid-western and north-western states
Canada = 13
USA = 347
All of the above seems to indicate the following:
 There are very few people by this surname (both spellings) in Canada and USA;
 In the USA, there are over twice as many KIELTY spellings as KEILTY;
 in Canada, there are almost 4 times more KEILTY than KIELTY spellings;
In general terms, it appears that most of this name (all spellings) first settled in eastern North America and later moved westward or to the south. Whether they were all from the same ancestry remains to be discovered. Where and when the spelling variations started is also waiting to be explained.
In general, the periods of greatest Irish arrival and settlement in North America were the 1820's, again after the potato famine of 1847, and again between 1890-1920.
Other listings on GENFORUM.COM say that many came from Wexford and Waterford in the south-east corner of Ireland. Because of that, a good guess is that many of them sailed to North America from Cork, Ireland (from the port of Cobh = Cove).