Slaiss: south-east of Cruachan, between Ord and Inneoin
Slane: a town on the Boyne, in Meath
Slechta: south-west of Kells, in Meath
Slemain Mide: "Slane of Meath," Slewen, three miles to the west of Mullingar, in Westmeath
Sliab Betha: Slieve Beagh, a mountain whereon the Counties of Fermanagh, Tyrone, and Monaghan meet
Sliab Culinn: Slieve Gullion, in the County Armagh
Sliab Fuait: the Fews Mountains, near Newtown-Hamilton, to the west and north-west of Slieve Gullion; in the southern part of the County Armagh
Sliab Mis: Slieve Mish, a mountain in the County Kerry, extending eastwards from Tralee
Sliab Moduirn: the Mourne Range, in the County Monaghan, partly in Cavan and partly in Meath
Sruthair Finnlethe: a river west of Athlone
Sualtaim (or, Sualtach) Sidech: the human father of Cuchulain
Suide Lagen: Mount Leinster, in the County Wexford
Tadg: a river in Conalle Murthemni
Taidle: near Cuib
Taltiu: Teltown, in the County Meath, on or near the Blackwater, between Navan and Kells; one of the chief places of assembly and burial of the Ulstermen
Taul Tairb: in Cualnge
Telamet: a river in Conalle Murthemni
Temair: Tara, the seat of the High King of Ireland, near Navan, in the County Meath (pronounced _Tavvir_)
Tethba descirt: South Teffia, a territory about and south of the river Inny, in the County Longford
Tethba tuascirt: south-east of Cruachan, in Teffia, County Longford
Tir Mor: in Murthemne
Tir na Sorcha: a fabled land, ruled over by Manannan
Tir Tairngire: "the Land of Promise"
Tonn Clidna: a loud surge in the Bay of Glandore
Tonn Rudraige: a huge wave in the Bay of Dundrum, in the County Cork
Tonn Tuage Inbir: "the Tuns," near the mouth of the river Bann on the north coast of Antrim
Tor Breogain: "Bregon's Tower," in Spain
Tromma: south-east of Cruachan; also the name of a river flowing into the Boyne near Slane
Tuaim Mona: Tumona, a townland in the parish of Ogulla, near Tulsk, south of Cruachan Ai, County Roscommon
Tuatha Bressi: a name for the people of Connacht
Tuatha De Danann: "the Tribes divine of Danu," the gods of the Irish Olympus
Turloch teora Crich: north of Tuaim Mona
Uachtur Lua: in the land of Ross
Uarba: a place in Ulster
Uathach: one of the three women-teachers of Cuchulain and Ferdiad
Uathu: north of Ochain
Ui Echach: the barony of Iveagh, in the County Down
Umansruth: a stream in Murthemne
Usnech: father of Noisi, Annle and Ardan
Uthechar: father of Celtchar and of Menn