I see I've fallen behind while waiting for my miserable internet (I'm with one of Canada's two giant ISPs, and it seems they have a problem that has been spreading countrywide since last week ...) -- but I'll go ahead and post what I've been puddling with in the interim.

One to consider.
Charlotte Harris, wife, married, 42, mariner's wife, born Strood, Kent, living in Gravesend, Kent. Her place of birth is a ditto to the neighbour's.
With her are a John Mason, 41, and Elizabeth Mason, 39, Elizabeth being sister of Charlotte, and John a labourer.
The household also has Elizabeth Harrington 13, niece of Charlotte Harris (so presumably a child of Elizabeth Mason from before her marriage to John Mason) and Mason children John 12, Harriet 6, Henry 3 and Charlotte 1.
One to think about. Unfortunately, I don't see a Harrington-Mason marriage, and if Elizabeth Mason had a previous marriage to Harrington, it would have been before registration. But if you really wanted to pursue that possibility, Charlotte Mason's birth was registered in Gravesend Sep quarter 1849 and that would give her mother's surname. Her christening shows her mother was Elizabeth Mary.
https://familysearch.org/pal:/MM9.1.1/N6KY-YHGThere is one Ryan in Strood in 1841: Michael Ryan, 61, born in Ireland, Army Pensioner, in a lodging house.
In 1851 in Milton near Gravesend, Kent, there are William Ryan born c1815 in Devonport and his wife Ann, with children born in London. He is a Customs something? Master (as is his neighbour, which I also can't read). That's possibly interesting. And of course there are numerous Irish and other Ryans of that and the previous generation in the vicinity, many probably with military connections.
Now, contradicting my Charlotte in 1841 theory, and searching for any Harris in Kent in 1851 who was born in Devon, we find Sarah H and another one at boarding school:
Thomas, 13, born Plymouth, at the Greenwich Hospital Schools in Greenwich. (The school is full of boys born in Plymouth.)
That would fit with being a child of Sampson and Charlotte.
However ... in 1838 there is a Thomas Harriss, 3, not born in county, at the District Greenwich Royal Hospital for Seamen, but it is identified as Union Workhouse and the children as pauper, so that could be the one in 1851 and he was just there all along, and that profile wouldn't fit with your Harris-s.