Mazur’s principle gives a criterion under which an irreducible mod \(\ell \) Galois representation arising from a modular form of level Np (with p prime to N) can also arise from a modular form of level N. We prove an analogous result showing that a mod \(\ell \) Galois representation arising from a stable cuspidal automorphic representation of the unitary similitude group \(G=\textrm{GU}(1,2)\) , which is Steinberg at an inert prime p, can also arise from an automorphic representation of G that is unramified at p.
