The memorial stone was for Harry Corlett, perhaps erected when his wife, Mary Fry, died in 1877. "Harry, the next born, emigrated to the United States in the '40s, was a prominent farmer in Warrensville till 1859, when he went to California, prospected and speculated, gathering a snug sum, and returned to his home in Warrensville." The "snug sum" no doubt helped pay for the fine stone http://www.io.com/~tcd/d0000/g0000086.html#I564
Even though he moved around, I would think he would have known when his mother died.
You are probably right about Daniel dying at son John's. John's first wife's surname was also Corlett. Could it have been her mother Margaret on the 1860 census?