Means 'Help (to the) foreigner' from the (Irish) Gaelic 'gall' = foreigner, and 'cobhair' or 'cabhair' = help. The clan were originally based in the south of Co Donegal (= fortress of the foreigner), now throughout Ireland and abroad. Descended from Niall of the Nine Hostages, High King and warlord/pirate, died A.D. 455. According to the Guinness Book of Records "The world record for the largest same last name gathering involved 1,488 participants with the surname Gallagher at an event held in Letterkenny, Ireland on 9 September 2007." http://www.gallagherclan.org/2007_gathering_index.aspx
http://www.gallagherclan.org/2007_gathering_index.aspx