This is the usual form of the name Elizabeth in Spain and Portugal, though elsewhere it is considered a parallel name, such as in France where it is used alongside Élisabeth. The name was borne by two Spanish ruling queens, including Isabel of Castile, who sponsored the explorations of Christopher Columbus.
As an English name, it became popular due to the Fleetwood Mac song Rhiannon (1976), especially in the United Kingdom and Australia.
As a Christian name, Ruth has been in use since the Protestant Reformation. In England it was associated with the archaic word ruth meaning "pity, compassion" (now only commonly seen in the word ruthless). The name became very popular in America following the birth of "Baby" Ruth Cleveland (1891-1904), the daughter of President Grover Cleveland.
As an English name, it was occasionally used during the Middle Ages in honour of the Old Testament heroine. It did not become common until after the Protestant Reformation, at which time it was often spelled Susan.