Dear Leslie Ann
I too have a John O'Donnell from Tipperary. Born c.1820-2 John and his brother James(bc 1823-25)O'Donnell put up their ages and came to Australia in 1841 aboard the "Eleanor".
They were the sons of John O'Donnell and Margaret Tobin (deceased) - both from Gammonsfield, Tipperary.
Now for the "foggy" side of the story. There is a family rumour that their father John O'Donnell (farmer and ex soldier) remarried "a french woman" after his wife Margaret died and they had another family. The older boys then took off for a better life in Australia.
Does this ring any bells for you?