Danu is a Goa'uld queen and is one of the oldest queens left alive.
She spawned most of the Goa'uld who impersonated Celtic gods, some of the most notable of whom were the System Lords Morrigan and Manannan mac Lir. Currently, she is quite mad, being almost entirely disconnected from reality. She has also become infertile, and Morrigan passes off her own larvae as Danu's. (RPG: "Living Gods: Stargate System Lords")