Lynch, which is today one of the most common surnames throughout Ireland, is unusual in that it has two completely distinct origins. The first is Norman, from de Lench, possibly derived from a French placename now forgotten. The family settled initially in Co. Meath, and a branch then established itself in Galway, where they rapidly became one of the strongest of the famous "Tribes of Galway"; between 1484 and 1654 no fewer than 84 mayors of the city were Lynches. One of their number, James Lynch, mayor in 1493, is reputed to have hanged his own son for murder when no one else could be found to carry out the sentence. The arms illustrated, a classic example of the simplicity of Norman heraldry, are for this family. In Ireland, the term "lynching" is generally believed to be derived from this James Lynch, although in the U.S. the likeliest origin is usually cited as the 18th-century American justice of the peace, Charles Lynch, who ordered extralegal punishment for colonial acts during the American War of Independence.
The second origin for the name is Gaelic, from the Irish Ó Loingsigh, grandson of Loingseach, meaning "seaman". Given the importance of the sea in Irish life, the surname naturally arose quite separately in a number of areas, including Clare/Limerick, Sligo, west Cork, Cavan, Donegal and the north Antrim/Derry region. In west Cork, the family were initially among the leaders of the Tuath Ó nDunghalaigh, based near the modern town of Clonakilty, and are recorded in the 16th and 17th centuries as chiefs of an area in the Beara peninsula, in the parish of Kilcaskan, under the overlordship of O’Sullivan Beare. Like O’Sullivan, they were dispossessed after the final collapse of the old order at the end of the 17th century. In the north, the Ó Loinsigh were also prominent, as chiefs of the old kingdom of Dál Riada in medieval times.
As the variety of geographical sources implies, the Gaelic origin is responsible for the wide frequency of the surname today. In addition the usual imprecision in the anglicisation of surnames has played a part; in many places Ó Loingseacháin and Mac Loingseacháin have been rendered as Lynch, while Ó Loingseacháin, Mac Loingseacháin and Ó Loingsigh have all at times been given as Lynch, Lynchy and even Lindsay.
. The best-known contemporary Lynch in Ireland is undoubtedly John Mary ("Jack") Lynch (1917 -), Taoiseach from 1966 to 1973 and from 1977 to 1979. He himself was born in Cork city, though his family originated in west Cork. He was, surprisingly, the first Corkman to become Taoiseach. Outside Ireland the name has also become well-known; David Lynch has directed some of the strangest and most intriguing films of recent years, such as Eraserhead and Blue Velvet, as well as the television classic Twin Peaks. Better known on Wall Street would be one of the largest U.S. investment banks, Merrill Lynch.