We have a life of St Samthann, which survives in three Latin manuscripts, the one in the Bodleian in Oxford being the most reliable. Dorothy Africa prepared an English translation, which is published in T. Head (ed.) Medieval Hagiography: An Anthology (Routledge, 2001).
She is recorded in the Martyrology of Oengus, and in the Martyrology of Donegal, both placing her feast day on the 19th December. This website contains a little more detail about her.