  In ferromagnetic superconductors, like URhGe, superconductivity co-exists
with magnetism near zero field, but then re-appears again in a finite field
range, where the system also displays mass enhancement in the normal state. We
present theoretical understanding of this non-monotonic behavior. We explore
the multi-band nature of URhGe and associate re-entrant superconductivity and
mass enhancement with the finite field Lifshitz transition in one of the bands.
We found good agreement between our theory and a number of experimental results
for URhGe, such as weakly first order reentrant transition, the dependence of
superconducting $T_c$ on a magnetic field, and the field dependence of the
effective mass, the specific heat and the resistivity in the normal state. Our
theory can be applied to other ferromagnetic multi-band superconductors.
