The Roman Catholic Archdiocese of Boston said it has placed a priest on an administrative leave of absence after receiving an allegation of child sexual abuse. The conduct by the Rev. Joseph F. Byrne, a retired senior priest, allegedly occurred in the early 1970s. It was only recently reported to the archdiocese, church officials said. The archdiocese said it immediately notified law enforcement officials of the allegation. It has also initiated its own preliminary investigation. Byrne was also the subject of an allegation in 2012, the archdiocese said. That conduct also allegedly occurred in the 1970s, according to the archdiocesan statement. That allegation was found to be unsubstantiated, officials said.