As noted above, the current anglicized name for the townland is Glendaduff, although the form Glanduff was used at times in the 18th and 19th centuries. You can see more about the townland at this link:
https://www.townlands.ie/mayo/gallen/attymass/attymass-east/glendaduff/
There are links there which you can use to see other info, such as Griffith's Valuation (mid-1800's) and the 1901 and 1911 censuses. In Griffiths Valuation, there were several Gallagher's listed as tenants there, as you can see at this link (and the following pages there). Thomas, Patrick, and Hugh Gallagher are each listed several times, along with a Denis Gallagher (some of the "duplicate" listings may represent the same person holding more than one tenancy):
https://www.askaboutireland.ie/griffith-valuation/index.xml?action=doNa…
If you look at the 1901 census link, you can see (starting at page 12 at the site to which it will send you) that there were still three Gallagher families living in the townland then, headed by Hugh, Thomas, and Bridget Gallagher (the last one presumably a widow holding the tenancy of her deceased husband). Unlike Griffith's Valuation, all of the family members are listed in the census.
The Gallagher/Gallaher surname is more common in the parishes to the south of Attymass, on down through Swinford (Kilconduff), Bohola, and Killedan (where my Gallagher grandfather grew up). Although the surname Gallagher is often associated with County Donegal (where there are still many Gallagher’s), scholars now believe that a separate Gallagher “clan” originated in the Mayo/Galway area. In Irish, Gallagher is Ó Gallchobhair (modern Irish spelling Ó Gallchúir), and it means “descendant of the foreign help”, referring to (real or legendary) mercenary warrior ancestors who were brought in from Scotland or elsewhere in Ireland to fight in local wars (known as a gallóglach, or “gallowglass”). It may also refer to Norse ancestors, since the term “gall” was used for them as well. As you may already be aware, the area remained Irish-speaking until very late (up to the 1920's or so in some areas). Although they were given anglicized (or sometimes Biblical) names in baptismal registries, people named Thomas, Hugh, Patrick, or Denis may well have been known to their families as Tadhg, Aodh, Pádraic, or Donncha, at least until the later 19th century.