'Rosabelle' is from 'The Lay of the Last Minstrel: Canto VI' published in 1805.
The places mentioned in the poem are near Edinburgh.
Saint Clair is sometimes pronounced Sinclair.
King James III bestowed Ravenscraig Castle to William Sinclair (St Clair), 4th Earl of Orkney in 1470.
Rosabelle and her entourage were drowned crossing the Firth of Forth.
Rosslyn Chapel, which won't have changed much since Sir Walter last saw it, is also the setting of the climax of the Da Vinci Code. It now attracts a huge number of visitors, presumably looking for the Holy Grail.