I can confirm that I have never sent or received official Departmental correspondence from an email address that is not either provided by the Houses of the Oireachtas or my Department.
My Department has an ICT systems usage policy that provides a suite of ICT policies governing the use of ICT systems by all users, with every effort being taken to obtain a balance between IT security and the usability of the Department systems. That policy includes a section on how limited use of personal email is permitted, subject to the conditions set out in the policy document.
As regards my advisers, since I became Minister in May 2016, there have been some rare occasions where my advisers have had to communicate through their personal emails on Departmental matters. This correspondence has included engagement on a number of topics, for example;
Representations from local representatives seeking meetings on DTTAS matters;
Update on changes to IT systems;
Photographs from DTTAS events;
Information about sporting events.