the book i am currently working on now is based in Las Vegas and i was wondering if anyone could help me out with something, what is the rule on using real names of places that are in other countries?
I don't think there are any, it's a country name, not like you're using copyrighted works. When referencing a city in a country you can say... London, England. I'd be specific when naming them, if you've got anything set in the United Kingdom make sure to specify either England, Ireland, Scotland, Wales, or Northern Ireland... Hope this helps...
I hope that it helps too, but just for the sake of accuracy, Ireland (by which you clearly mean the Republic of Ireland, since you've listed Northern Ireland separately), is not part of the United Kingdom.
there isn't any 'rule' other than using the correct capitalization... if that's not what you meant, then what aspect of naming real places are you unclear about?...