Actually, Seanán is more like Shan-AWN. This is because
Sean is said as
Shan. With the Shan-AWN pronunciation you can see how
Shannon comes from Seanán.
Senan or Senán would not be Shan-an or Shan-awn or because "Sen" is said differently to "
Sean".
Senan would be Sen-AN and Senán would be Sen-AWN. It's usually spelt
Senan though(no fada) and pronounced accordingly.