It means either FAIR HERO or WHITE WARRIOR
Early records of the name McGinley in Scotland mention Gilaspyh M'Kinley, who witnessed a charter to Archibald, earl of Argyll in 1493. Malcolm M'Inley was denounced as a rebel in Balliwilling in 1675. William McKinley was twenty-fifth president of the United States (1897).
Mag Fhionnghaile — the classical spelling of the surname McGinley in its native language, Irish. The form that has been used for hundreds of years in Co Donegal and is still used today.
Mag Fhionnaile — this is the standard modern spelling in Irish. Both this and the older form are in current use. These both derive from the earlier (pre 1500 a.d.) obsolete Irish Gaelic forms of Mag Findgail, Mag Findgaile and Mag Findgoil.
The surname derives from the very old Irish words fionn meaning ‘fair haired’ and gal meaning ‘valour’. The full meaning of the surname is therefore ‘son of the fair haired valorous one’. The Gaelic ending gal is usually interpreted in a warrior sence. The originator of the McGinleys (whoever he may have been), most likely was a fearless warrior. Most Irish surnames usually start with Ó or Mac. In our surname, Mag is an occasional variant of Mac that is found with surnames following with an aspirated/silent 'F' or a vowel. The letter F is silent (as is the last G in the older Gaelic spelling). The pronunciation for the Irish form is Magunaluh or Maginaluh (both said quickly). The Mag surname forms are predominantly found in Ulster (Antrim, Armagh, Cavan, Derry, Donegal, Down, Fermanagh, Monaghan and Tyrone). Historically, in the northern half of Ireland, there has been a tendancy to soften the hard sound of the letter ‘c’ in Mac allowing the variant Mag to develop. Mag surnames can be found in some parts of north Conacht that are close to Ulster (or had large Ulster settlement after the 1600's) such as Leitrim, Mayo, Roscommon and Sligo. Mag surnames are very rare in other parts of Ireland and do not occur at all in Gaelic Scotland.
For a more detailed report on the meaning of the McGinley surname, you are advised to read: The McGinley Clan. Volume 1; Warriors and Holy Men. Author: Proinsias Mag Fhionnghaile (Contact Us).