A name used for both males and females, but more common for males.
From the Irish surname, Gaelic O Ryan, means descendant of Rian. This names has gone through a number of different forms, probably starting out as something like Ó Maoilriain, and moving through Ó Riagháin, shortened to Ó Riain, and fianlly morphing into moder form. 'Rian' originally ment king. (From the Gaelic 'Ri').
In modern times Ryan is quite common as a first name for males, but only very rarely applied to females. This is somewhat counter the usual trend of names to become more feminine over time -- witness Vivian, Shirley, Courtney and Leslie, all of which used to be masculine names.
Ryan became a popular name for boys in the 1970s, and has not left the top 100 list of most common boys names since then.