at the end could be the last remains of the Celtic magus 'plain' or 'market', or it is a simple latinization, and the origin is just the Celtic word *uerno, alder tree (Breton gwern, Welsh gwern, Irish fern, modern French verne).
æt ðɪ ˈɛnd kʊd biː ðə lˈæst ɹᵻmˈeɪnz ʌvðə kˈɛltɪk mˈæɡəs plˈeɪn ɔːɹ mˈɑːɹkɪt, ɔːɹ ɪɾ ɪz ɐ sˈɪmpəl lˌætɪnᵻzˈeɪʃən, ænd ðɪ ˈɔːɹɪdʒˌɪn ɪz dʒˈʌst ðə kˈɛltɪk wˈɜːd ˈæstɚɹˌɪsk jˈuːənˌoʊ, ˈɔːldɚ tɹˈiː bɹˈɛtən ɡwˈɜːn, wˈɛlʃ ɡwˈɜːn, ˈaɪɹɪʃ fˈɜːn, mˈɑːdɚn fɹˈɛntʃ vˈɜːn.