Richard Woods's
Hispanic First Names gives Siria as a feminine short form of
Ciriaco.
José Albaigés Olivart's
Diccionario de Nombres de Personas lists Sira, Siriardo, Siricio, Sirico, Sira and Siro as Spanish given names. It is possible that Siria is a deliberate alteration of one of them.
But I think it's most likely that this is simply a direct transfer from the place name, as Siria seems to be the normal spelling of the name of the country which is Syria in English.