Looking for my Irish Roots

My maiden name is Kelly, and I know that my great grandfather's name was Nathan Kelly.  I do not know if it was he or his father that ended up in Ohio, but I do know that he came to the United States by way of Canada.  I am not sure of the date either, except that he emigrated from Ireland sometime in the mid 1800s.  I was always told that he was the second son of an English lord with the surname Heathington.  This has never rang completely true to me.  I believe that the truth may be that he was the illegitimate son of an English Lord in Ireland, and his immigration to Canada was related to the Great Starvation.  Any information that may aid my search for the truth would be appreciated.