Gallagher (Old Irish: Ó Gallchobhair; Ó Gallchobhoir; Modern Irish: Ó Gallachóir) is an Irish Gaelic clan based most prominently in what is today County Donegal. The clan name originated in the 10th century as a derivative of its founder Gallchobhair mac Rorcan. They descend in the paternal line from the Northern Uí Néill's Cenél Conaill. The immediate progenitors of the Gallaghers held the High Kingship of Ireland during the 6th and 7th centuries in the form of Ainmuire mac Sétnai, Áed mac Ainmuirech, Máel Coba mac Áedo and Cellach mac Máele Coba; Áed was also an ancestor of the O'Donnell and O'Doherty clans. Indeed, during the Middle Ages, the Gallaghers were Marshals of the Kingdom of Tyrconnell, ruled by their previously junior kinsmen the O'Donnells.

In Ireland the anglicised version of the name – "Gallagher" is pronounced "Goll-a-her" in its native Donegal, while elsewhere in the country it is most frequently pronounced as "Gall-a-her". Outside Ireland there has been a corruption or alteration of the pronunciation, resulting in "Gall-ag-er" in some parts of Britain and the USA. The name Gallagher is an anglicisation of the Irish surname Ó Gallchobhair,Ó Gallchobhoir (or two newer spelling forms: Ó Gallchóir and Ó Gallachóir,), these being masculine forms; the corresponding feminine forms are Ní Ghallchobhair (newer forms Ní Ghallchóir and Ní Ghallachóir), and means "foreign assistance" or "foreign helper". Apart from the aforementioned spelling there are at least 30 recorded variants including Gallacher, , , Gallocher, Galliher, Gallaugher, Galagher, Galegher, Goligher, Golliher, Gollaher and Gallahue.

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