Twitter trends location?

Hey guys I was wondering how twitter locates local trends. Is it based on ip of the user or the country which the user is from?

For instance if a thing gets lots of attention from people who are using say an Irish ip, but their registration details say they are Canadian. Will it show as a Canadian or an Irish trend?

Tweets are geo tagged based on IP or location services etc. so no where the account is registered doesn't matter.