Sow-en. It's an Irish Gaelic word and many of the letters are pronounced differently and then again after different letters. Similar with other words and names such as Siobhan, which is pronounced nothing like it looks like it should be.

After vowels m is usually a fricative, but sometimes a (nasal) stop, in which case it is also often written double:

dám /daːṽ/ "company"

lom or lomm /lom/ "bare"

I found some interesting links about the Gaelic language on Wikepidea.

http://en.wikipedia.org/wiki/Samhain