Hello I'm looking for a marriage in Ireland probly county Monaghan of a Hugh Gallagher to a Ann ? He was born about 1825 and Ann anout 1828 
Civil registration of Catholic marriages began in 1864 and from 1845 for non-Catholic marriages. You can have a look at the online civil registration index (
www.familysearch.org) but with a common name like Hugh Gallagher and not knowing the wife's maiden name it might be impossible to find them if there was a marriage listed (if a Catholic ceremony before 1864 it won't be there).
Any children born in Ireland after 1864? mother's maiden name will be on birth certificate.
Any children marry or die outside Ireland/England/Wales? mother's name may be listed on marriage or death certificate.