Translation according to P. W. Joyce:
Tormaun in Galway; corruption (by metathesis) of Trommán, the elder or boor-tree. For Tromm. See vol. i. p. 517 [reproduced below].
The elder-tree. The elder or boortree is called tromm or trom, gen. truim [trim]. The best known place named from this tree is Trim in Meath, which was so called from the elder-trees that grew near the old ford across the Boyne; it is called in the Book of Armagh Vadum-Truimm, a half translation of its Irish name, Ath-Truim, the ford of the boortrees, of which only the latter part has been retained. We have numerous names terminating in -trim and -trime, which always represent the genitive of trom; Galtrim in Meath, once a place of some importance, is called in the annals, Cala-truim, the callow or holm of the elder; Gortvunatrime near Emly in Tipperary, the gort or field of the bottom-land (bun) of the elder. The old name of the mountain now called Bessy Bell, near Newtownstewart was Sliabh-truim (Four M.), the mountain of the elder. A place where elders grow is often called tromaire [trummera], from which Trummery in Antrim derives its name; it is shortened to Trummer, as the name of a little island in the Clare part of the Shannon; and in Wexford it takes the form of Trimmer. Tromán, a diminutive of tromm, meaning either the elder-tree or a place producing elder, has given name to Tromaun in Roscommon, to Tromman in Meath, and to Trumman in Donegal.