  We study a self-adjoint realization of a massless Dirac operator on a bounded
connected domain $\Omega\subset \mathbb{R}^2$ which is frequently used to model
graphene quantum dots. In particular, we show that this operator is the limit,
as $M\to \infty$, of a Dirac operator defined on the whole plane, with a mass
term of size $M$ supported outside $\Omega$.
