I was watching season 3 of the HBO series "Deadwood" tonight on DVD. I'm nearly certain that in the first episode three "Cornish" miners are actually speaking Irish. Likewise in the second episode there is a "Cornish" miner being questioned by the sheriff via an interpreter. He clearly says "Bhi se marbh" at one point.

Does anyone know if Cornish and Irish share that degree of similarity or is it perhaps that the producers were using Irish thinking no one would notice?

If you choose to check this out yourself, be forewarned...the english is riddled with profanities.

Cornish and Irish are both Celtic languages, but Cornish belongs to a different branch of Celtic (Brythonic) than Irish does (Goidelic). It shares similar syntax and even grammatical peculiarities (like prepositions inflected for person) to Irish but any cognates you would find would not be 'close' cognates. For example in Welsh the word for 'son' is 'Map' whereas in Irish it's 'Mac'.

Cornish would be completely unintelligible to an Irish speaker, unlike a language like Scottish Gaelic which would prove intelligible to an Irish speaker with only minimal effort.

So I'm going to go with "The producers were using Irish hoping no one would notice." :P

Wouldn't be the first time when the producers haven't done their homework and didn't know that they are dealing with different languages. It would have been more credible to just make the miners Irish unless their Cornishness was really that important.

Yes. It's like Icelandic and English, say. Almost impossible that people would understand each other.

The word "dead", though, may be understood, it is "marow" in Cornish and "marbh" in Irish, pronounced more or less the same way.

He was dead = Marow o ef ; Ef o marow ; Yth o ef marow.
Nothing like "bhí sé"...

That's what I thought. Looks like the producers dropped the ball on this one!

and that there were no more monoglot cornish speakers after the 1770s, or so I've read...

More like no Cornish speakers PERIOD after the 1770s. According to Ellis in The Cornish Language (1974), the last monoglot Cornish speaker died in 1676.

The language was already in decline in the 16th century. There's a debate as to whether the last 'traditional' [bilingual] native speaker died in 1777, or if a handful survived into the 1800s.

Even more confirmation that the producers have been caught napping. Unfortunately, the series has been discontinued so there's no one with whom to raise the issue.

Years ago I read about some kind of adventure movie that was made way back when, and there came a scene where the explorer or his guide came across a group of natives off in the jungle somewhere, and one of them had to speak to them in their own language. The actor in question happened to be an Irish speaker, so, that's what he used to address them.