Saoirse is the daughter of Arthur Weasley and Iona Crow. Crow, who was a co-worker of Arthur, slipped a love potion into his drink to make him fall in love with her. Although the potion wore off, Iona ended up prego and gave birth to Saoirse. Iona dropped Saoirse off at an orphanage shortly after giving birth where Saoirse was adopted by the MacLachlans. No one in the Weasley family knew about this until she showed up at Hogwarts :D
Saoirse is a year younger than Ginny.
