Mah-v-awn
* Mah: Like "ma" in "mama," but slightly longer.
* v: A voiced bilabial fricative, like the "v" in "van," but it's a subtle sound and some might pronounce it more like a "w" sound. The "gh" in the middle is a lenited (softened) "g," producing this sound.
* awn: Like the "awn" in "dawn," but possibly slightly shorter. The final "n" is a nasal sound.
The stress is on the first syllable. The sound is quite fluid; it's best to hear a native speaker pronounce it to get the exact nuances. You can search YouTube for "Mathghamhain pronunciation" to find audio examples.