It is probably derived from an Anglo-Saxon locational name, [æt þǣre] sīdan īege, "[at the] wide island/watermeadow" (in the dative case).
[1] There is also a folk etymological derivation from the French place name Saint Denis.
In 1768, Dudley Cosby, Minister Plenipotentiary to Denmark between 1763 and 1765, was made Baron Sydney (second creation), of Stradbally in the Queen's County, in the Peerage of Ireland.
Thomas Townshend was created Baron Sydney (third creation) of Chiselhurst in 1783.
Elizabeth, daughter and heiress of Thomas Sidney, fourth son of Robert Sidney, 4th Earl of Leicester, was the grandmother of John Shelley-Sidney, whose son Philip Sidney (1800–1851) was raised to the Peerage of the United Kingdom as Baron De L'Isle and Dudley.