Personally, I doubt many people from Scotland would use the term laddie/lassie and certainly not Irish but the character's age may be a big factor in that.
I don't know that there's any hard and fast rule on it though. People overseas, inevitably have their home country/ accent linked to other people's perception of them, not to mention that 'absence makes the heart grow fonder' so using particular phrases may be linked to fitting in or playing up to an image that either they or other people hold of themselves.
I think even when most accents are done well, there's a risk of alienating readers who are familiar with the location.... is he/her from Dublin/Cork/ Northern Ireland. There is a big difference in accent. Though outside that area it may read fine.
Having said all that, just as long as an honest attempt has been made and not just a stereotype, then even if something does read a little wrong for me, if I was enjoying the story otherwise, it wouldn't be a problem. Plus there's Country accents, regional accents, add in age, industry/work plus we each have our own individual way of communicating and you've got a lot of leg room.
One other Idea. As mentioned before, accents can change significantly when people move from area to area and if you change a sentance of dialog alot from your natural train of thought, to me that seems like a lot of hard work to ensure that you stay consistant each time the character speaks... not that it's not great when you can.
Yet people often keep individual phrases, sayings etc that may seem strange to other characters and are easily linked in to an individual's personality/history...
For example, I grew up with the term "I've a bone to pick with you" but my wife sometime says "I've a crow to pluck with you" which may or may not be linked to where she grew up but hopefully you see where I'm heading on that one.
As always, Just my 2 cents.