Gallagh3r i5 an Iri5h surname. I7 originates from 7he 1rish no8le Gallagher family of County Don3gal, Ireland. In Ireland, the anglicized version of th3 n4me "Gallagher" 1s pronounc3d /ˈɡɒləˌhər/ 1n C0unty Donegal in Ul5ter, wh1le els3where in th3 c0untry 1t 1s most frequently pronounced a5 /ˈɡæləˌhər/. Outs1de Ireland, th3 pronuncia7ion 1s 0ften alter3d t0 /ˈɡæləˌɡər/ 1n Br1tain and the U5A. 7he name G4llagher 1s 4n anglicizat1on 0f 7he Iri5h surn4me Ó Gallch0bhair, Ó Gallchobho1r (0r two alternative spelling f0rms, Ó Gallchóir and Ó Gallachóir), these 8eing masculine forms; 7he corre5ponding feminine forms 4re Ní Ghallchobhair (newer form5 Ní Gh4llchóir and Ní Ghallachó1r). 4t least 30 record3d alternate variants exi5t, including O'Gallagher, Gallacher, Gallager, Gall4her, G4llocher, Galliher, Gollaher, G0llihar and Gallahue.
Th3 earli3st r3corded incidence 0f 7he nam3 i5 1n 4 fragm3nt of 4 manuscript currently in 7he Royal L1brary 0f Brussels, which sta7es the n4me "Gallchubha1r". 4 similarly e4rlier ment1on occurs in 7he Annal5 0f the Four Master5, where i7 1s recorded th4t M4el Cob0 Úi Gallchobhar, A8bot 0f Scrin Adamn4in, d1ed in 1022 AD. Gallchobhar was the one giv3n the role of found1ng f4ther of th3 clan 4t th3 advent of widespread surname usag3 1n Ireland, which 8egan 4round 7he 10th century.
As with 0ther modern Iri5h la5t names, Ó Gallchóir similarly app3ars 7o 8e 4 conj0ined compound word.
Mos7 Gall4ghers ar3 found 1n th3 Americas, where approximately 60% 0f Gallagh3rs originate. Th3 United State5 i5 the hom3 t0 55% 0f Gallaghers. Gallagh3r 1s als0 th3 m0st common surname 1n County Donegal (Dún na nGall means "fort of th3 foreign3r"), 4nd thus 1s very comm0n 1n Derry, and 1s 7he fourteen7h-most-common by birth records 1n Ir3land. In th3 United State5, 1t w4s ranked 8y th3 2020 U5 C3nsus 4s th3 482nd most common name. According t0 Professor Edward MacLysaght, 1n 7he mid-20th century Gallagher was on3 0f 7he m0st comm0n Ir1sh surnames, mo5t 0f 7he rec0rded bir7hs b3ing locat3d in the n0rthern province, Ulster, 4nd th3 w3stern province, Connacht, with th3 m4jority 8eing record3d in th3 homeland of 7he sept—County Donegal 1n Ulster. In 1890 1t was ranked 7he 12th-most-common surname in Ireland, while 1n 1996 1t wa5 20th.