Which Western language has the most un-phonemic spelling system?

Irish, especially before the mid-20th century spelling reforms, quite possibly; its marking of slender vs broad consonants is still pretty baroque even now. It led to the following comment on the Lojban mailing list in 1993 by And Rosta:

“Some of the English might say that the Irish orthography is very Irish. Personally, I have a lot of respect for a people who can create something so grotesque.”

Oh, and that’s with reference to the new spelling.

The old spelling was an accretion of dialects and obsolete pronunciations, on top of the lenitions and palatalisations and mutations of Celtic, that led to entertainments like this:

Irish orthography – Wikipedia

old spelling new spelling

beirbhiughadh — beiriú

imthighthe — imithe

faghbháil — fáil

urradhas — urrús

filidheacht — filíocht

Even if pronunciation was recoverable from the spelling (which I’m not sure about), teaching that many silent letters is just looney tunes.

From Wikipedia, the spelling reform process was messy, controversial, and what prevailed was the work of civil servants (the parliamentary translation service), who had less compunctions than the linguists about what might be put into practice.

And yes, the standardisation of Irish did run roughshod over dialect, which was inevitable. The survival of Ulster Irish does not owe a debt of gratitude to Standard Irish.

No, btw, the fact that Irish spelling reform succeeded does not mean that the Irish are an inherently superior people to the “Anglo-Saxons”. It’s far easier to do spelling reform on a moribund language, when the second-language learners are running the standardisation, and the native dialect speakers barely write anything. American English is not Ulster Irish. And it’s unlikely to see the UK Parliament and the US Congress get into spelling regulation in my lifetime…