  We derive an efficient method for treating renormalization contributions at
two-loop level within the functional renormalization group in the one-particle
irreducible formalism for fermions. It is based on a decomposition of the
two-particle vertex in charge, magnetic and pairing channels. The method treats
single-particle and collective excitations in all channels on equal footing,
allows for the description of symmetry-breaking and captures collective mode
fluctuation physics in the infrared. As a first application, we study the
superfluid ground state of the two-dimensional attractive Hubbard model. We
obtain superfluid gaps that are reduced by fluctuations in comparison to the
one-loop approximation and demonstrate that the method captures the
renormalization of the amplitude mode by long-range phase fluctuations.
