Hi again
For information, the family at The Carpet Mill, Creetown in the 1851, Ref given previously are:
Alexander Marshall, 46, tinsmith, b. Withorn Wig
Jane Marshall, 43, tinsmith's wife, b. Kilmaurs, Ayr
Jane, 20, tinsmith's daughter, b. Kirkmichael, Ayr
Alexander, 18, - b. New Luc, Wig
Robert, 13, b. Kilmaurs, Ayr
Elizabeth, 9, b. Portpatrick, Wig
Sarah, 7, b. Leswalt, Wig
Francis, 4, b. Stonykirk, Wig
Stewart, 1, b. Wigtown, Wig
Your great great grandmother, Jane Marshall was married to John williamson at Gatehouse of Fleet on July 1st 1855, according to the Rituals of the Catholic Chirch marriage (after Banns). As it is 1855, there is far more detail on this certificate

Both are shown as living at Back Fleet Street, Gatehouse. John was 25 and an Ag Lab and a widower and Jane was 23 and a hawker of Japanese Ware (?) and a spinster. John had 2 living children. John was born in Norther Ireland - location and date not known and, as you said, Jane born but not registered 20th August 1831, Kirkmichael, Ayrshire. Jane;'s parents were Alexander Marshall (whitesmith) and Jane, maiden name Marshall.
When you say, what happened to them afterwards, are you looking for information about the whole of the family from census and BMD information or something specific?
As they were tinsmith/travellers it might be worth putting a second query up on the Travelling People Board:
http://www.rootschat.com/forum/index.php/board,387.0.htmlRegards
Gadget