The more general statement that is true is the following:

> Let $X \subset \mathbb{C}^d$ be an irreducible affine variety defined by real polynomials. If $X$ has a smooth real point, then $X(\mathbb{R})$ is Zariski dense in $X.$

A sketch of the proof and some good examples are given here: *[Real Algebraic Geometry for Geometric Constraints][1]* by Frank Sottile (page 8).

The main reference is: *[Real Algebraic Geometry][2]* by Bochnak, Coste, Roy.

Fun facts: 

 1. If you ask instead for the density of $\overline{\mathbb{Q}}$ then that is true more generally for any $d$–dimensional affine variety $V$ defined over $\mathbb{Q}$ by Noether Normalization.
 2. If you remove the assumption that there is a smooth point, then the statement is false as $V(x^2+y^2)$ has only one $\mathbb{R}$-point which is not smooth, namely $(0,0)$, and $x^2+y^2$ is irreducible over $\mathbb{R}$ and the $\mathbb{C}$-locus is dimension 1 (so the $\mathbb{R}$-locus not Zariski dense).

  [1]: https://arxiv.org/pdf/1606.03127.pdf
  [2]: https://www.springer.com/us/book/9783540646631