Thanks CuriousDiana and Scotmum for your replies.
Wm Carolan married in Scotland in 1856, so no clues there I'm afraid.
His age from the 1851 census would have been about 26 and he was a shoemaker, single and living with 3 other single shoemakers as a lodger in Edinburgh, so I suspect he hadn't been there long.
I have just discovered that there is a Donegal in Cork and also in Tipperary, but the 1851 census just said Ireland and the 1861 census said Donegal, Ireland, so I am even more at a loss.
Anyway, thanks once again for your interest.
Lilibet