sue2566 on Family Tree Circles

I have been researching James Shanahan/Shannon married to Mary Scully. Both were from Ireland, married and lived in Nelson, Northumberland, New Brunswick, Canada. I cannot get beyond these two people, where they came from in Ireland. I was told Moses Scully and Maragaret McNamara Scully were the parents of Mary Scully Shanahan. I was also told that James changed his name to Shanahan from Shannon. I have plenty of information on this family while in New Brunswick which I am willing to share. I need to place them in Ireland. I have no idea what ship they came on, James and Mary married August 21, 1843 in New Brunswick. On the 1852 canada census they came to Canada at different times. He says 1838 and she says 1830.
The story goes that they eloped from Ireland because her father felt he was below her station, that was why he changed his name. How can this be true if they arrived 8 years apart?
Any help would be greatly appreciated.