Its not up to the CAA to say what you can do in Ireland on a UK issued licence; that's up to the Irish CAA.
Licences are not ICAO compliant, its the State that issues it that complies with ICAO!
Yes you can have the Microlight rating included in your licence but wait for it; the CAA may not have any software to do this as nobody told licensing about the January Law Change!
(5) Whenever a reference is made to aeroplanes this does not include microlights as defined nationally, unless otherwise specified.
But that does not mean you can't fly one on a SEP class rating! A microlight fits within the SEP definition of an aeroplane with a single piston engine, whatever else you call it doesn't matter, it simply means you can't count the hours on such a device towards aeroplane requirements.