The Irish people (Irish: Muintir na hÉireann or na hÉireannaigh; Ulster-Scots: Airisch or Airish fowk) are an ethnic group who originate in Ireland, an island in northwestern Europe. Ireland has been populated for around 9,000 years (according to archaeological studies, see Prehistoric Ireland). The Irish people's earliest ancestors are recorded in legends - they are claimed to be descended from groups such as the Nemedians, Fomorians, Fir Bolg, Tuatha Dé Danann and the Milesians. Lebor Gabála Érenn, a book of Irish mythology tells that Tuatha Dé Dananns were Scythian descendants.
The main groups that interacted with the Irish in the Middle Ages include the Picts, Scots (themselves of Irish origin), and the Vikings. Due to this contact, Icelanders are noted for having some Irish descent. The Anglo-Norman invasion of the High Middle Ages, the English plantations and the subsequent English rule of the country introduced the Normans and Flemish into Ireland. Welsh, Picts, Bretons, and small parties of Gauls and even Anglo-Saxons are known in Ireland from much earlier times.
There have been many notable Irish people throughout history. The 6th century Irish monk and missionary Columbanus is regarded as one of the "fathers of Europe", followed by Kilian of Würzburg and Vergilius of Salzburg. The scientist Robert Boyle is considered the "father of chemistry". Famous Irish explorers include Brendan the Navigator, Robert McClure, Ernest Shackleton and Tom Crean. By some accounts, the first European child born in North America had Irish descent on both sides; and an Irishman was the first European to set foot on American soil in Columbus' expedition of 1492.
Large populations of people of Irish ethnicity live in many western countries, particularly in English-speaking countries. Historically, emigration has been caused by politics, famine and economic issues. An estimated 50 to 80 million people make up the Irish diaspora today, which includes Great Britain, the United States, Australia, Canada, Argentina, Chile, Jamaica, Trinidad, South Africa, New Zealand, Mexico, France, Germany and Brazil. The largest number of people of Irish descent live in the United States-about ten times more than in Ireland itself.
However, it had been recognised that the estimated numbers of the Irish diaspora may be highly inaccurate; one reason is that the majority of ancestral censuses conducted within the United States and Canada record self-reported ancestry, which is often unreliable.  The majority of people living within immigrated populations (i.e. Australia, United States, Canada etc.) are of mixed ancestry due to decades, at times centuries, of inter-marriage with other immigrants or indigenous populations, hence claiming one specific ancestry is often personal preference or perceived ancestry rather than fact.The author Jim Webb suggests that a large number (he suspects near half) of those claiming Irish-American ancestry, especially among Protestants, may be Scots Irish (that is, descendents of Scottish people who populated the Irish province of Ulster).
The Irish were among the first people in Europe to use surnames as we know them today. It is very common for people of Gaelic origin to have the English versions of their surnames beginning with "O'" or "Mc" (less frequently "Mac" and occasionally shortened to just "Ma" at the beginning of the name).
"O'" comes from the Gaelic Ó which in turn came from Ua, which means "grandson", or "descendant" of a named person. Names that begin with "O'" include Ó Briain (O'Brien), Ó Cheallaigh (O'Kelly), Ó Conchobhair (O'Connor), Ó Domhnaill (O'Donnell), Ó Cuilinn (Cullen), Ó Máille (O'Malley), Ó Néill (O'Neill), Ó Sé (O'Shea), Ó Súilleabháin (O'Sullivan), and Ó Tuathail (O'Toole).
"Mac" or "Mc" means "son". Names that begin with Mac or Mc include Mac Diarmada (MacDermott), Mac Cárthaigh (MacCarthy), Mac Domhnaill (MacDonnell), and Mac Mathghamhna (MacMahon, MacMahony, etc.). Mac is commonly anglicised Mc. However, "Mac" and "Mc" are not mutually exclusive, so, for example, both "MacCarthy" and "McCarthy" are used. While both "Mac" and "O'" prefixes are Gaelic in origin, "Mac" is more common in Scotland and in Ulster than in the rest of Ireland; furthermore, "Ó" is far less common in Scotland than it is in Ireland. The proper surname for a woman in Irish uses the feminine prefix ní (meaning daughter) in place of mac. Thus a boy may be called Mc Domhnaill whereas his sister would be called Ní Domhnaill.
A son has the same surname as his father. A female's surname replaces Ó with Ní (reduced from Iníon Uí - "daughter of the grandson of") and Mac with Nic (reduced from Iníon Mhic - "daughter of the son of"); in both cases the following name undergoes lenition. However, if the second part of the surname begins with the letter C or G, it is not lenited after Nic. Thus the daughter of a man named Ó Maolagáin has the surname Ní Mhaolagáin and the daughter of a man named Mac Gearailt has the surname Nic Gearailt. When anglicised, the name can remain O' or Mac, regardless of gender.
There are a number of Irish surnames derived from Norse personal names, including Mac Suibhne (Sweeney) from Swein and McAuliffe from "Olaf". The name Cotter, local to County Cork, derives from the Norse personal name Ottir. The name Reynolds is an Anglicization of the Gaelic Mac Raghnaill, itself originating from the Norse names Randal or Reginald. Though these names were of Viking derivation some of the families who bear them appear to have had Gaelic origins.
"Fitz" is an old Norman French variant of the Old French word fils (variant spellings filz, fiuz, fiz, etc.), used by the Normans, meaning son. The Normans themselves were descendants of Vikings, who had settled in Normandy and thoroughly adopted the French language and culture. With the exception of the Gaelic-Irish Fitzpatrick (Mac Giolla Phádraig) surname, all names that begin with Fitz - including FitzGerald (Mac Gearailt), Fitzsimons (Mac Síomóin/Mac an Ridire) and FitzHenry (Mac Anraí) - are descended from the initial Norman settlers. A small number of Irish families of Gaelic origin came to use a Norman form of their original surname-so that Mac Giolla Phádraig became Fitzpatrick - while some assimilated so well that the Gaelic name was dropped in favor of a new, Hiberno-Norman form. Another common Irish surname of Norman Irish origin is the 'de' habitational prefix, meaning 'of' and originally signifying prestige and land ownership. Examples include de Búrca (Burke), de Brún, de Barra (Barry), de Stac (Stack), de Tiúit, de Faoite (White), de Londras (Landers), de Paor (Power). The Irish surname "Walsh" (in Gaelic Breathnach) was routinely given to settlers of Welsh origin, who had come during and after the Norman invasion. The Joyce and Griffin/Griffith (Gruffydd) families are also of Welsh origin.
The Mac Lochlainn, Ó Maol Seachlainn, Ó Maol Seachnaill, Ó Conchobhair Mac Loughlin and Mac Diarmada Mac Loughlin families, all distinct, are now all subsumed together as MacLoughlin. The full surname usually indicated which family was in question, something that has being diminished with the loss of prefixes such as Ó and Mac. Different branches of a family with the same surname sometimes used distinguishing epithets, which sometimes became surnames in their own right. Hence the chief of the clan Ó Cearnaigh (Kearney) was referred to as An Sionnach (Fox), which his descendants use to this day. Similar surnames are often found in Scotland for many reasons, such as the use of a common language and mass Irish migration to Scotland in the late 19th and early to mid-20th centuries.