Richard de Clare (d.1090), the son of Gilbert, count of Brionne, accompanied William the Conqueror to England in 1066. He took his new title fromthe fief of Clare in Suffolk. Richard descendants acquired the earldom of Gloucester by marriage, and became the leading barons of the south-easternMarch by early in the 13th century.
http://www.castlewales.com/clares.html