Shane McGowan's accent is a mystery to me.

He went to Holmewood House prep school (not far from me in Speldhurst in my 4 years living in Kent in the 70s) and to Westminster School from there. That (~to me) doesn't really conjure up images of a sort of say rough London accent.

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Pre Covid, I used to meet up with a few people from Strabane in a local N Norfolk pub (landlord is from there) for a musical weekend. They just sound "Northern Irish" to me.

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On an another bit, I only remember one person getting my county of birth right. He was the postmaster at the post office/shop in Conwy, N Wales. He (apparently from Shropshire) reckoned my own Shropshire bit (muddled with longer periods then in N Wales twice and Kent) was quite distinctive...