Well, do you? Let’s hear Emma (no, not her, another one, but I did choose this video for a reason) say it.
In other words:
The standard Irish pronunciation is “sow-in” with the “ow” like in “cow.” Other pronunciations that follow with the many Gaelic dialects include “sow-een” “shahvin” “sowin” (with “ow” like in “glow”).
So… the Latin transcription for English speakers was done by a Welsh completely wasted on Scotch, then?
Never try to understand how gaelic words came to be spelled the way they are. That way madness lies.
For instance, bh is clearly a V as in Siobhan. GUESS AGAIN because mh is V in Niamh! It makes zero sense. I lived in Scotland for 8 years, during which time I learned the hard way to just smile, nod, and pretend everything is fine. (Presumably it has something to do with confounding the English, but I’m not certain on that theory)
After many years of research (by which I mean trying to talk to Irish relatives and Scottish residents of Appalachia and Nova Scotia) I can confirm that both the Scottish and Irish languages do in fact exist solely to confuse the English. After many years of additional exhaustive research (I sat on the couch and drank a beer while looking up funny Welsh words on the internet) I can confirm that they also possess a language based around the vexation of Englishmen.
The English, true to form, responded to this flagrant provocation with the creation of Cockney Rhyming Slang, a language that cannot be understood by anyone. Including the people actually speaking it.
Then Americans got in on it. Well, the Cajuns anyways.
I gave up on languages the day I heard an englishman pronounce worcestershiresauce 
Asked him where all the leftover letters went…but never got a straight answer o.O
I think that’s because the Cajuns mostly originated from the regions of Canada infested with Scots, Irish and Frenchmen, and thus, ‘confounding the British’ nearly became a national sport.
Avoid Massachusetts, then, for the sake of your sanity!
Particularly Worcester (pronounced Woosta’)
Haverhill (pronounced Haevril)
And Gloucester (pronounced Glosta’).
Well you’re gonna hate these:
Happisburgh - pronounced ‘haysbra’
Alnwick - pronounced ‘annick’
Fowey - pronounced ‘foy’
Rampisham - pronounced ‘ransham’
and of course Beaulieu, which manages to eschew both English and French pronuniciation by being said “byou-lee”.
England is the best 
If we’re talking place names, we English are particularly self-loathing in that regard. We’ve got the original mispronounced Gloucester and Worcester. As well as aliceh’s examples, we also have, for example:
Belvoir - “beaver”
Marylelone - “marlibone”
Norwich - “norritch”
Salmesbury - “sarmsb’ry”
Oswaldtwistle - “ozzeltwissel”
And just to catch you out, there’s a bakery chain called Greenhalgh’s, whose website proclaims it as “A name you can trust.” Good bakery, but don’t trust the name - it’s pronounced “Green-alsh’s”.
There are countless more examples. We’re a strange lot. We’ll complain about others’ pronunciations, while our own make zero sense either.
So do Sahuagin celebrate Samhain?
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch
The ultimate headwrecker. I won’t even try to pronounce it. 
Properly pronounced ‘Bob’.
It is surpassed in name length only by Taumatawhakatangihangakoauauotamateaturipukakapikimaungahoronukupokaiwhenuakitanatahu (also pronounced ‘Bob’) New Zeland.
Even as a welsh speaker that one can be tough 
The double set of “LL” towards the end always catches me out