English Bob Theory

I didn’t catch anything in the movie that would have suggested he wasn’t English.

Well you're nearly right. He does put on an accent but he is English.
Most of his dialogue is delivered in an upper class "rich English accent" the idea as you rightly suggest is to increase his credibility and give the impression of being a member of the British aristocracy, hence the book title "Duke of death" but like the book itself all is not as it seems.
In the lines that you notice the accent drops he lapses into a lower class cockney\London type accent which suggest a more common upbringing.





"A wise man proportions his belief to the evidence." David Hume

So you're saying Clint Eastwood hired an Irish actor to impersonate an American impersonating an Englishman. That is rich.



He killed sixteen Czechoslovakians. Guy was an interior decorator.

His accent when he’s leaving town is totally different to the one he used before though, which I thought was a great touch, from upper class aristocracy to low class cockney, another show of how his facade had been lifted.