In my excessive reading of Harry Potter fanfiction, I've often been astounded to find, in the middle of stories, a note from the author complaining that "Hagrid's accent is too annoying to write, so I'm not going to." It's astounding mostly because I'm perfectionist enough that I cannot understand releasing a half-finished story, not when something so glaringly obvious is still undone; but, as well as that, I'm astounded because Hagrid's accent just isn't very hard.
It's nowhere near as bad as the still-legible Scottish of, say, James Kelman or Irvine Welsh, e.g.
They aw went tae fuckin Cambridge or wherever, n ye dinnae git in thaire unless yuv goat brains. Ye kin bet they didnae start daein silly walks n aw that shite in thir exams.J.K Rowling's Hagrid-speech is more about the illusion of local colour. It might be irritating to write Hagrid in character -- fawning over Dumbledore, using phrases like "thumping good one", and so on -- but there's very little to his accent.
(Irvine Welsh, "Glue", p. 41)
I collected all of his dialogue from "Philosopher's Stone" through "Goblet of Fire", roughly nine and a half thousand words (thank heavens for e-book piracy!), and took a good look at it. The conclusion is that his accent, such as it is, consists of pronouncing "for" as "fer", "to" as "ter", "you" as "yeh" and "your"/"you're" as "yer"; dropping the "g" from the "-ing" suffix; and occasionally dropping the "t" from words like "just". Using "summat" for "something" is by far the most interesting change; and that's pretty much it.
I'm not sure how many of the discrepancies are OCR error, but it seems that Rowling isn't very consistent. Of the 250-odd instances of the word "to", for example, just under 200 of them were accented into "ter". The other 50 were left as regular ol' tee-oh.
Anyhow, as a public service, I offer this little Hagridizer script. It's a five-minute job, to be sure, and can't correct any serious defects -- Hagrid will sound out-of-character talking about stochastic calculus no matter how much you accent it -- but hey, at least it'll be consistent.