now why in SamHill would they pronounce it that way?

Because Gaelic uses a different orthography than English. The Celtic languages have a morphophonological process called mutation (or sometimes lenition). In the earlier native alphabet this was marked with a dot over the letter affected, but in mordern orthography this is marked with an aitch.

Here's a chart:

b ~ bh => /b/ ~ /w/
d ~ dh => /d/ ~ /ɣ/
g ~ gh => /g/ ~ /ɣ/
m ~ mh => /m/ ~ /w/
p ~ ph => /p/ ~ /f/

Somhain is accented on the second syllable. Hope that helps.