  We show that the bar version of the $\mathrm{Pin}(2)$-monopole Floer homology
of a three-manifold $Y$ equipped with a self-conjugate spin$^c$ structure
$\mathfrak{s}$ is determined by the triple cup product of $Y$ together with the
Rokhlin invariants of the spin structures inducing $\mathfrak{s}$. This is a
manifestation of mod $2$ index theory, and can be interpreted as a
three-dimensional counterpart of Atiyah's classic results regarding spin
structures on Riemann surfaces.
