  We consider Riemannian 4-manifolds $(X,g_X)$ with a Spin^c-structure and a
suitable circle bundle $Y$ over $X$ such that the Spin^c-structure on $X$ lifts
to a spin structure on $Y$. With respect to these structures a spinor $\phi$ on
$X$ lifts to an untwisted spinor $\psi$ on $Y$ and a U(1)-gauge field $A$ for
the Spin^c-structure can be absorbed into a Kaluza-Klein metric $g_Y^A$ on $Y$.
We show that irreducible solutions $(A,\phi)$ to the Seiberg-Witten equations
on $(X,g_X)$ for the given Spin^c-structure are equivalent to irreducible
solutions $\psi$ of a Dirac equation with cubic non-linearity on the
Kaluza-Klein circle bundle $(Y,g_Y^A)$. As an application we consider solutions
to the equations in the case of Sasaki 5-manifolds which are circle bundles
over Kaehler-Einstein surfaces.
