Father Woulfe in his book ‘Sloinnte Gaedheal is Gall’ (Gaelic and non-Gaelic Surnames) has the following entries against ‘Guerin’:

“Ó Géaráin – O Gerane, O Gieran, Geran, Gearon, Guerin, Gearn, Gearns, Sharpe; ‘des of Géarán’ (dim. of géar, sharp); the name of a family of Uí Fiachrach, originally seated in the barony of Erris, Co. Mayo, but long dispersed; also the original name of the family of Mag Fhionnbhairr, or Gaynor.

Mag Uidhrín – Maguirin, M’Gwyrin, M’Guiverin, Magiverin, Magivern, Magiveran, MacGiverin, MacGiveran, MacGivern, Guerin; ‘son of Uidhrín’(a dim. of Odhar; vid. Mag Uidhir); an old Ulster surname. Early in the 12th century, Eachmarcarch Mac Uidhrin was chief of Cinel Fearadhaigh, in the present Co. Tyrone. In the 16th century, the name was peculiar to Co. Down, and even at the present day is confined to that county and the neighbouring counties of Antrim and Armagh.”

The book ‘Surnames of Ireland’ by Edward McLysaght has the following entry against the names:

“(Mac) GAYNOR          (O) GERANE, Guerin

All the records agree in placing the territory of Mac Fhionnbhair, chief of Muntergeran, on the west side of Lough Gowna in the present county of Longford. Muntergeran is a shortened anglicized form of Muintir Geradhain and the eponymous ancestor of the family of Gaynor, or MacGinver as it was formerly more phonetically rendered, was Fionnbhair (Finbar) Ó Geradhain, who was lord of that area in the eleventh century. Writing a century ago O'Donovan found the normal anglicization then to be Maginver with the synonym Gaynor already coming into more general use. The Gin sound is preserved in the form MacGinty found synonymous with Gaynor in south Ulster. The latter, however, was no innovation as it appears as a principal name in Co. Westmeath in the "census" of 1659. Westmeath and Cavan, which both adjoin Co. Longford, are the counties in which the name is chiefly found today. The prefix Mac, as is often the case with names beginning with a vowel or aspirated F, becomes Mag, first in speech and later in the written word, so that Mag Fhionnbhair is now the normal form in Irish.
Woulfe identifies the Ó Geradhain mentioned above with Ó Géaráin, a Hy Fiachrach sept located in Erris (Mayo), and regards MacGinver (Gaynor) as an offshoot of it.

This sept of Ó Géaráin appears to be now almost extinct. There was another sept of the same name, anglicized O'Gerane and later Geran, which is listed among the principal names in that county in 1659. This survives there today as Guerin. Some of our Guerins, however, may be of quite different origin, Guerin being a French Huguenot surname.”