Ireland has not signed up for Schengen because, to do so, would require the setting up of border controls at the NI border - something it would be politically impossible to propose - and a very retrograde step socially, communally, and economically.  However  Ireland would love the UK to sign up for Schengen as well as the Euro and the Charter of Fundamental rights so that North South differences and potential anomalies can continue to reduce.

notes from no w here