The subject of my search was a man named John Patrick Nolan. John Patrick Nolan went by both John Nolan and Patrick Nolan, which was the first obstacle in my search. In 1901 John Nolan was residing with his first wife, Margaret in Bellevue Buildings, Usher’s Quay Ward in Dublin City. By 1911 he was residing with his second wife, Sarah and the children of his first and second marriages. The only child born to his second marriage was Annie or Anna Mary.
The birth registration, found online at www.irishgenealogy.ie identified Anna Mary’s mother as Sarah Nolan, formerly Brennan and a search of the marriage index at www.irishgenealogy.ie found the marriage of a Patrick Nolan and Sarah Brennan in 1905, a year before Anna Mary’s birth.
On inspection of the marriage registration, I found that Sarah was recorded as Sarah Cronin, a widow, the daughter of Michael Brennan, a butcher. The problem is that the civil marriage index at www.irishgenealogy.ie did not also list the marriage under the name Sarah Cronin. This is not an accurate record of the original indexes. A similar search at www.findmypast.ie and www.familysearch.org found Sarah Cronin and Sarah Brennan both indexed under the same year, registration district, volume and page number, which matched with Patrick Nolan.
This means that if you are searching for the second marriage of a widow at www.irishgenealogy.ie her marriage may only be indexed under her maiden name, rather than her married name. It might be sensible to also search the free marriage indexes online at www.familysearch.org where you will discover whether an entry found in that index is missing from www.irishgenealogy.ie It is entirely possible that other second marriages indexed at www.irishgenealogy.ie are indexed under the married name of the bride, but if you cannot find what you are looking for, try searching alternative indexes.