You seem to be confusing two different Robert Clucas' one born in Castletown c1826 and one born in Marown c1827.

1841 Castletown Census - John Clucas and Catherine Clucas (nee Quilliam) living Douglas Street with their children John 20, Jane 15, Robert 15, Margaret 13.

1851 Castletown Census - John and Catherine Clucas living 131 Douglas Street with their children Jane and Margaret. Their son Robert is a shoemaker living two doors away at 133 Douglas Street with his wife Catherine.

1871 and 1881 Andreas Census - Robert Clucas a shoemaker from Castletown and his wife Catherine living in Ardonan.

1881 Patrick Census - The other Robert born c1827 Marown living with his wife Margaret.

Unfortunately to further muddy the waters the Robert from Castletown also married someone from the Lace family - Catherine Lace on 15th June 1848 Andreas.