Noetherianity of a quadratic integer order
= Noetherianity of a quadratic integer order
For a nonsquare integer $d$,
$$
\mathbb Z[\sqrt d]\cong\mathbb Z[X]/(X^2-d).
$$
The integers are Noetherian, the Hilbert basis theorem makes $\mathbb Z[X]$ Noetherian, and quotients preserve Noetherianity.