This is the Gaelic word for "freedom". It's pronounced SEER-sha. This word is often used as a name and a term with political overtones, especially by those who believe that Northern Ireland should be part of Ireland, and not the U.K. It has growing popularity as a girl's name, usually indicating strong Irish pride.
A delightful hostel owner in Ireland who fiercely insisted that Northern Ireland should be part of Ireland named his dog Saoirse.