Hi Jim,
I wouldn't exclude the Braddan one, and three entries in Douglas if you include the 4-yr-old, but the one Frances wrote about who was living in Fancy Street, Douglas, looks likely because the head of household was John Skillicorn, mariner (thanks, Mary, for the lookup!). Also the witness was John Skillicorn (brother?).
1841 census Conchan, Douglas, Fancy Street:
John Skillicorn 39 mariner
Cathr 33
John 10
Wm 2

He is in the Ancestry index under "John Skellicorer".

There were two John Skillicorns bapt Douglas who could have fitted the 10-yr-old with father John and mother Cath (both bapt 1831). One Cath Christian, and the other a different Cath, no surname given.

Sue