From the experts:
Connected with the Latin word meaning 'senator,' this started as a nickname, used as a term of respect. St. Senan was a famed founder of monasteries, whose ruins can still be seen. Legend has it that before he could found it, he had to banish a great monster. Other versions are Sionan, Sinan, Synan and Sinon. Seanan is also a pet form of Sean.