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McLoughlin is one of nearly two dozen Anglicisms for three Gaelic-Irish surnames: Mac/Nic Lochlainn (most commonly Anglicised McLaughlin, Ó/Ni Máoilsheáchlainn (usually McLoughlin or M'Loughlin), and Mac/Nic Lochnaigh (usually rendered Loughney but occasionally McLoughlin or MacLoughlin).
Mac and Ó are masculine prefixes; Ni/Nic is feminine. Mac is rendered into English as Mac, Mc and M'. Ó is rendered O' . The feminine prefix is rendered into English according to its masculine counterpart (e.g. Nic na Mhara > McNamara, Ni hEaghra > O'Hara). They are used with names that originated as patronymics. Occasionally, the prefix Mael/Maol is Anglicised Mac/Mc.
The surname Mac/Nic Lochlainn originated in Ulster. Lochlann was originally a name for Scandinavia, especially Norway, so called after the fjords - Lochlainn, the adjectival form, literally means "lake-ish" or "full of lakes," a "land of lakes," etc. The Annals of Ulster refer to the Vikings as Lochlannach meaning "of Lochlann." 
Lochlan(n)/Lochlain(n) is still used as a man's personal name in Ireland and Scotland, though not commonly. English forms of the surname Mac/Nic Lochlainn are concentrated in western Ulster, on both sides of the international boundary. But the family are found throughout Ulster, and spread eastward during the Middle Ages, across the North Channel into Scotland where they became the Clan MacLachlan. Spellings of the name peculiar to Scotland include McLachlan, McLachlin, and McLauchlan. The Mac/Nic Lochlainn and their Scottish cousins are descendants of the Northern Uí Néill. 
The Ó/Ni Máoilsheáchlainn surname originated in Meath. The family claim descent from High King Máel Sechnaill mac Domnaill of the Southern Uí Néill, relatives of the Northern family. The surname was originally rendered O'Melaghlin in Norman French and Middle English but was corrupted to McLoughlin and M'Loughlin in the 18th century. 
The Lochnaigh family in Connaught, who claim descent from the Uí Fiachrach dynasty, mostly became Loughney in English but are very occasionally called McLoughlin. They tend to be concentrated in Galway.
MacLoughlin of Cineál Eoghain
- Domnall Ua Lochlainn, reigned 1083–1121, with opposition.
- Muirchertach Mac Lochlainn, reigned 1156-1166.
Ó Máoilsheáchlainn of Clann Colman
The Ó Máoilsheáchlainns of the Kingdom of Mide (presently the counties of Meath, Westmeath, and parts of the counties of Dublin, Kildare, Offaly, Longford, and Louth, all now in Leinster, and part of County Cavan, now in Ulster) are descendants of the Southern Uí Néill.
High Kings of Ireland of this family included:
- Conchobar mac Donnchada, reigned 819-833, with opposition.
- Máel Sechnaill mac Máele Ruanaid (aka Máel Sechnaill I), reigned 846-862.
- Flann Sinna, reigned 879-916.
- Donnchad Donn, reigned 919-944.
- Máel Sechnaill mac Domnaill (aka Máel Sechnaill II), reigned 980-1002;1014-1022.
The Ó Máoilsheáchlainns of Meath lost their lands and their power in the centuries following the Norman Invasion - their homeland of Meath even losing its status as a kingdom/province and being absorbed into Leinster. The family were recorded as O'Melaghlin, and after the 17th century, McLoughlin.
The Northern Uí Néill family expanded from Ulster into Argyll, in Scotland, where Middle Irish 'Mac/Nic Lochlainn' became Modern Scots Gaelic 'Mac/Nic Lachlainn', most commonly spelled 'MacLachlan in Scots and English.
See the Clan MacLachlan article for more information.
- Alan McLoughlin
- Coleen Rooney (née McLoughlin)
- Colin McLoughlin
- Collin McLoughlin
- Cornelius McLoughlin born July 11, 1897 a descendant of Máel Sechnaill mac Domnaill
- Denis McLoughlin
- Emmett McLoughlin
- James McLoughlin
- Dr. Jean-Baptiste/John McLoughlin ("Father of Oregon")
- John McLoughlin (9/11 attacks survivor), also featured in the film World Trade Center
- Maurice McLoughlin
- Paul McLoughlin
- Tom McLoughlin
- William McLoughlin, footballer (Burnley FC)
- Tony McLoughlin TD
- Keith McLoughlin
- Seán McLoughlin
- Seán William McLoughlin , Irish YouTube game commentator (also known as Jacksepticeye)