Sencha macAilella: the wise counsellor and judge of the Ulstermen
Sered: a plain in the north of the barony of Tirhugh, County Donegal
Setanta: the real name of Cuchulain
Sid: the terrene gods (pronounced She)
Sil: in Lecale, in the County Down
Sinann: the river Shannon
Siuir: the Suir, a river in Munster, forming the northern boundary of the County Waterford
Slabra: a place north of Selaig, near Kells, in Meath
Slaiss: south-east of Cruachan, between Ord and Inneoin
Slane: a town on the Boyne, in Meath