Irish Gaelic pronunciation is rather difficult to learn, and for me harder to remember. Needlessly modest NW is as expected correct.

1.Samhain (Celtic)
... in the pagan calendar. Samhain may be pronounced in a number of ways but the most
common pronunciation is "sow-in" (sow rhymes with cow). The modern day ...
http://www3.kumc.edu/diversity/ethnic_relig/samhain.html