The Nest thermostat with Electric Ireland is the 3rd generation. It would not be possible to install a different version. You could get two 3rd gen Nest thermostats for €400 in total if you opt for a second Nest.
We recommend contacting our colleagues in the Home Energy Services department directly on 1850 372 333 (9am - 6pm, Mon - Fri) to discuss this further. They also have an email address. We can provide general information here but we recommend calling for any technical queries you may have.
Please let us know if there is anything else we can help you with.