Shirai must be her maiden name. The 1995 soundtrack was her first credit. We didn't get the kanji 清水 until nearly 10 years later in 2004. A lot can change in that time.
There's a 白井多永子 on Facebook:
https://www.facebook.com/profile.php?id=100010707814018
She has only 3 friends added: one is Yoshiko Itoh (伊藤予示子), who also performed on the Ghost in the Shell. The other is a musician named Yasuyuki Shimizu (清水康幸). That's no coincidence.