Certain surnames identify Irish Catholic ancestry, but don't establish a dominant genealogy. Someone's last name tells you that somewhere along the line there was an Irish Catholic ancestor, but not that the contemporary person with that surname is either Catholic, Protestant or, for that matter, Irish.

If Michael Houlihan immigrated to the U.S four generations ago from predominantly Catholic Southern Ireland (now The Irish Republic), his fourth generation descendant may be as little as 6.25 percent genetically Irish. As has long been claimed, the U.S. is a melting pot of every ethnicity in the world.

While Michael Houlihan's name sounds Irish and although he may have come from Dublin in the Irish Republic, that doesn't mean necessarily he's Catholic or even that his ancestors were predominantly Catholic. Persons in Ireland, like persons everywhere, pick up and move. Michael Houlihan may be the descendant of a MacDonough, a name with Scotch origins, who migrated from predominantly Protestant Northern Ireland to predominantly Catholic Southern Ireland and then married a Houlihan. Nevertheless, some surnames make it somewhat likely that somewhere along the line there was an Irish ancestor who was probably Catholic.

Ireland is a little over 300 miles in length, running North to South. Geographically speaking, most of Ireland is within the predominantly Catholic Irish Republic. Northern Ireland occupies only about a sixth of the island in the Northeast corner. Despite a history of troubled relations between the two countries going back centuries, persons do intermarry and so, in those areas of the Irish Republic closest to Northern Ireland, it's harder to tell from the last name whether the family is Catholic or Protestant. Because, until the 20th Century a 300-mile journey was taxing and time-consuming, the farther south one goes, the more likely it is the inhabitants are Catholic, or at least Catholic in origin.

Here's a list of the 26 counties in the Irish Republic along with the surname most often appearing in the historic 1890 survey of Irish Catholic parish registers for each county:

• carlow: Murphy
• cavan: Reilly
• clare: MacMahon
• cork: O'Sullivan
• derry: Doherty
• donegal: Gallagher
• dublin: Byrne 
• galway: Conealy (also commonly spelled Conneely and Connolly)
• kerry: Sullivan
• kildare: Kelly
• kilkenny: Brennan
• laois: Delaneh
• leitrim: Kelly
• limerick: Ryan
• longford: Reilly
• louth: Byrne
• mayo: Walsh
• meath:Reilly
• monaghan: Duffy
• offaly: Kelly
• roscommon: Kelly
• sligo: McLoughlin
• tipperary: Hogan
• waterford: Power
• westmeath: Lynch
• wexford: Murphy
• wicklow: Byrne      

Although most of the popular names in the list are recognizably "Irish sounding," many other popular names in the 1890 registry, which draws exclusively from Catholic parish birth records, have origins that are Scottish, Welsh, English, Norse, Norman and Flemish. It's not only the U.S. that's a melting pot.