James Doohan has said that he based Scotty's accent on a Scottish man he knew during WWII. Scotty is characterized as being "an Old Aberdeen pub crawler". Aberdeen and the surrounding area is known for a dialect called Doric, a form of Scots. This accent isn't heard often in media and it makes sense that a lot of people wouldn't be familiar with it. Less people speak it these days and those that do tend to tone down their accent when speaking with others not from the area. I have heard people say that Scotty sounds Irish to them but this could be because Scots is also spoken in Northern Ireland (Ulster Scots). In my opinion Doohan does a good job with his accent. Scotty does use some strange phrases such as "The haggis is in the fire for sure" but that's more of a writers issue. Doohan also does a number of different voices for TAS and it's not really surprising that he was able to do a fair impersonation of someone he knew. His son's accent is Star Trek Continues is also pretty good. Interestingly, Simon Pegg based his version of Scotty's accent off of his wife's, who is from Glasgow, which gives the character a different feel as the Glasgow has a rougher reputation than Aberdeen. It also changes the character's backstory from the East Coast to West Central which is never addressed. Pegg's accent is very good. I'm interested to hear the opinions of other people familiar with Scottish regional accents.