The EU is Northern Ireland's biggest trading partner

Press quote (Belfast Telegraph)
John Springford
09 May 2016
From 
http://www.belfasttelegraph.co.uk/opinion/debateni/the-eu-is-northern-irelands-biggest-trading-partner-yet-here-we-are-preparing-for-a-referendum-on-whether-to-stay-in-it-34695773.html

As the Centre for European Reform explains, and I quote: "In order to maintain access to EU markets, a Britain on the outside would have to sign up to many of the EU's rules. As a non-participant in the EU’s institutions, it would have little say over the rules."