You may have got this information by now but in the Irish civil registration index at www.familysearch.org
there is a marriage for a John Lynn Ballina registration district Year 1923 quarter Apr-Jun volume 4 page 417 and matching references for a Barbara Gallgher that looks like a good fit. You can order the cert by downloading an application form at www.groireland.ie
if you order a photocopy it costs less and gives the same information - fathers' names for bride and groom and residence of both. From this you may be able to find John Lynn in the 1911 census.
Also there are some Lynn births registered in the Ballina district from 1930-1948 with mother's surname given as Gallagher- the index does not show mother's surname earlier than 1928 or so.
Hope this is of help.