[Rinnwald]

Somehow I doubt the Irish brogue was adopted for stereotype pirates as late as 1950?

As Katana said, it was Cornish, not Irish.

Jon C. Hopwood on IMDB:

His portrayal of Long John Silver and of Blackbeard, the Pirate (1952) created a persona that was so indelible that his vocal intonations created the paradigm for scores of people who want to "Talk Like a Pirate."

[the OlLine Rebel]

I’m not talking about the movie. Just the stereotype. I don’t know Cornish but it sounds incredibly like Irish, and there could be a reason for that.