The problem is that there are more sounds than we have letters for on the one hand (and us dumb dummies even have more letters than we have sounds - I'm looking at you, C/K/Q/S which combine for 2 different sounds), and specific letter combinations for other sounds that maybe should be their own letters (hey ch, th, sh, how's it going?).
So you try to approximate a different language, with different sounds, using english letters. I remember from New Zealand that 'wh' and 'f' are both used for Maori words, and sounded pretty much the same as each other to me, but seemed like they didn't sound the same to them.
So for the Samhain example - it might sound like "sowan" or something to us, but one thing I remember from watching PBS one time is that there is an actual physical/developmental issue where you can't hear the distinction between some sounds that aren't distinct in the language you grow up speaking. Without knowing anything about it, I'd imagine that the letters 'mh' were assigned to some Gaelic sound that is similar to but distinct (to a Gaelic speaker) from the 'w' sound.