the 1851 census def has 6m age for James -if he was born in 1846 then Saml's wife would have been abt 15 - rather young even for Irish catholics (the Manx generally looked to marry at 21 or a little older for the females)
Catholic practice was for baptism within days of the birth - the Manx Museum has one of the mixed registers for Catholic bapts + marriages but it only goes upto 1848 and I have photos - no 1846 entry there - there may be a later book - will ask tomorrow when in museum tho I thought all catholic bapts were transcribed and on www.imuseum.im - the main RC church was in Douglas with a travelling priest to Ramsey etc (see www.manxnotebook.com under parishes) so it is possible an entry was forgotten about or not entered in the main book
It does look like the same James - Stuart and steward/stewart sound the same if they were semi illiterate the form would be filled in by the enumerator - what you see in the census photos pre 1901 is the final 'tidied up' form by the enumerator