The name "Seine" comes from the Latin Sequana. Some have argued that Sicauna is cognate to the name of Saône River. However, a suggested relationship to the River Shannon in Ireland is unlikely, given the very different forms of the two. (Gaelic an tSiona, dative Sionainn, is from Prehistoric Irish *Sinona. Another proposal has it that Sequana is the Latin version of the Gaulish Issicauna Lower-Icauna, which would be the diminutive of Icauna, which was the Gaulish name of the Yonne River.) Some believe the ancient Gauls considered the Seine to be a tributary of the Yonne, which indeed presents a greater average discharge than the Seine. (The river flowing through Paris would be called Yonne if the standard rules of geography were applied.)
According to Pierre-Yves Lambert, a specialist of the Gaulish language, Sequana retains QV, that is unusual in Gaulish, which is normally a P-Celtic language, but he compares with the month name EQVOS, read on the Coligny Calendar. The name of the Gaulish tribe Sequani derives from it