Irish Americans are citizens of the United States who can trace their ancestry to Ireland. A total of 36,278,332 Americans—estimated at 11.9% of the total population—reported Irish ancestry in the 2008 American Community Survey conducted by the U.S. Census Bureau. Roughly another 3.5 million (or about another 1.2% of Americans) identified more specifically with Scotch-Irish ancestry. The Irish diaspora population in the United States is roughly 6 times the modern population of Ireland.

The only self-reported ancestral group larger than Irish Americans is German Americans. The Irish are widely dispersed in terms of geography, and demographics. Irish American political leaders have played a major role in local and national politics since before the American Revolutionary War: eight Irish Americans signed the United States Declaration of Independence, and 22 American Presidents, from Andrew Jackson to Barack Obama, have been at least partly of Irish ancestry. (See "American Presidents with Irish ancestry" below.)

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, 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.