The UK, Ireland, isle of man, etc are outside schengen.. and in their own common travel area called, 'the common travel area'.Ireland isn't part of the Schengen area, is it ? In which case the hard frontier is between Ireland and the EU which, in theory I suppose, could be sold as a solid enough border control provided it's well controlled Not sure about the first point though.
Not all of the CTA is inside the EU