Don't confuse Carthy/Swarbrick's Byker Hill with a regular Slip-jig (sorry, but for me a regular 9/8 is a slip jig, Irish or not). Their tune - which is actually one phrase of the Northumbrian Dorrington Lads - is a split rhythm: 12,12,12,123 not a regular 123,123,123. It's rather like saying the Bonny Black Hare is 4/4 when it's actually 8/8 played as 123,12,123. If you really want some fun, take two players and float a 9/8 melody over a waltz 3/4 backing - wonderful!
Tom