The name alludes to Sionna, a goddess in Irish mythology, meaning "possessor of wisdom". She is the namesake and matron of Sionainn, the River Shannon. The Sionainn is the longest river in the British Isles. The River Shannon, (Modern Irish: Sionainn), rises at Tiltinbane in the Cuilcagh Mountain and flows into the Atlantic Ocean in the Shannon Estuary. In the 2nd AD, Ptolemy gave the name “Sēnu” to the river, which O”Rahilly reads as “Senā” and translates “the Ancient” (Goddess). It is based on Old Irish “sen”, meaning “old, ancient”. As Ó hÓgáin explains, the original name of the river must have been “Senunā”, a word meaning “the old honoured one” It was written as Sinann and Sinand in Old Irish, and Sionann in Classical Irish.