No, I hadn’t heard of her either. We have a weekly college eucharist here in St Mary’s, organised by our student society, and I was celebrating this week. Although it was the last of term, I did not want a Christmas theme, so I glanced at a couple of lectionaries I had handy for readings. One of them noted it was the feast of St Samthann yesterday, although other resources place her today.
She was the adopted daughter of an Irish king, and like so many of the female Irish saints was delivered from an arranged marriage by a miracle and then devoted herself to serving God as a nun. Various miracles are recorded in later years, but she was known mainly for her wisdom: she gave guidance and advice to many, including the teacher Dairchellach and Maelruin, the leader of the Ceile De, one of the most significant reform/renewal movements of Irish Christianity.
The hagiographies do not mention it, but one other point I noticed: in more than half the stories I found recorded about her, she was laughing (‘giggling’ at one point, although I haven’t checked the Latin!).
She died in 739. A woman who taught the leading churchmen of her day, and whose holy wisdom repeatedly broke into laughter. I wish I had heard of her earlier.