Accent/dialect diversity in Ireland and Britain are mainly due to hundreds of years when no-one but the very rich travelled anywhere. Peasants and working people in Somerset, for example, had very little to do even with their neighbours in Dorset, while folks way over in Kent may as well have been in another country for all that anyone in Somerset ever saw of them. So quite small areas could develop distinct accents as most people never went more than 30kms from their birthplace. There’s a story that in around 1793, at the height of fears about the French Revolution, the Cumbrian poets William Wordsworth and Samuel Coledridge went on a holiday down to Devon. The locals soon reported them to the district magistrate as they thought they were French spies because they ‘talked funny’!
There are differences in the Aussie accent but they’re quite subtle and the bigger differences are between city and country people rather than from one state to another. On average Queenslanders speak more broadly and South Australians the most refined, and us Victorians have the flattest tone, but there’s not a lot in it. Being a young country, the accent’s also still in a state of change, and the biggest differences are probably generational. So compare Alf Stewart with fellow former Home and Away star Chris Hemsworth – both obviously speak with an Australian twang, but Chris’ is softer, and that’s becoming more the case as more people become city-dwellers rather than living on the land, and as our population diversity significantly increases.
But overall it’s definitely hard to differentiate. Having lived for 2 years in England and Scotland, plus watched a lot of British TV, I can pick out where someone is from there far easier than I can here at home. At the very least I can usually say whether an English person or Scot is from the north or south of each country from their accent, and some of them are really obvious – like folks from Liverpool, Newcastle and Glasgow. Ireland I’m not as familiar with. People from around Dublin I think are reasonably distinctive, maybe simply because there’s relatively a lot of them, and ones from down south around Cork, but otherwise I can sometimes hear the differences but couldn’t say who was from where.