Of course none of the accents in that clip are supposed to be Glaswegian...they're all meant to be from Edinburgh (though Robert Carlyle was born in Maryhill in Glasgow). For the full Glaswegian effect, try this bit of Rab C Nesbitt, parodying the fact that previous series of this show had been shown subtitled in England
on the other hand we from Glasgow don't seem to have problems understanding accented US stuff (eg The Wire)...I guess because we're brought up on imported TV. The Glasgow accent is nothing though compared to full-on Geordie and Aberdonian dialects, with lots of non-English vocabulary.
A Glaswegian here, who learned phonetics for a text-to-speech project. A Glasgow accent actually has the closest correspondence between spelling and pronunciation of any accent of English.
We don't all speak like Rab C. Nesbitt. The reason you might find it difficult to understand some Glaswegians is not so much lack of familiarity with their accent, but because they don't speak Standard English. What you're hearing is a mixture of English and a variety of Scots. Scots (https://en.wikipedia.org/wiki/Scots_language) is a distinct language, though it's very closely related to English. So when, for example, you hear a word like mair (meaning more), it's not a different accent, it's a different language, just like German mehr or Dutch meer. Glaswegians pronouncing "more" in Standard English don't sound that different from Canadians or most Americans.
Thanks for the correction - it's years since I've seen "Trainspotting" and somehow I relocated it to Glasgow in my mind; that said, Wikipedia helpfully informs that it was actually mostly shot in Glasgow!
"The Wire" is an interesting example since two of the protagonists (McNulty and Bell) are played by English actors (the former an Old Etonian, just like our prime minister!) putting on Baltimore accents; there's even a delightful scene where McNulty attempts a deliberately dreadful English accent.
You are also correct about Geordies - I lived in Newcastle for a number of years and never really understood the locals except by way of sign language.
https://www.youtube.com/watch?v=Ud9LT7R60kU#t=6m30s
on the other hand we from Glasgow don't seem to have problems understanding accented US stuff (eg The Wire)...I guess because we're brought up on imported TV. The Glasgow accent is nothing though compared to full-on Geordie and Aberdonian dialects, with lots of non-English vocabulary.