Well your county of residence is UK as that is where you live. Ideally you would want to use an ID document that matches that which you may or may not possess.

However if you were to put down Ireland using your passport as the linked ID then you wouldn’t be restricted to just 2 days use in Britain.

If I don’t intend to use the interrail pass at all in the UK then would it be best to say I am resident in Ireland and use my Irish passport, or just say I live in the UK but have an Irish passport?

I am trying to anticipate any hassle of Brexit consequences during my actual travel around Europe - If I am resident in the UK how does that affect my European citizenship if at all and how best to communicate this when booking or when using the pass?

Interrail pass and it’s use has nothing to do with Brexit or EU, it is a rail ticket nothing more.

Immigration officers have no interest in people’s rail tickets while rail staff are only interested in rail tickets and their validity.

It is required that you associate a passport or other document with your interrail pass primarily for identification purposes as each pass is only allowed to be used by one person.

Perfect thanks for clearing that up! Was getting ahead of myself but better to be safe than sorry - All the best!

For clarity - the only significant issue is that an Interrail global pass is restricted to a maximum of 2 travel days where you can use your pass in your country of residence as freely as if you were in any other participating country. 

Strictly for Interrail purposes residents of Northern Ireland are restricted to 2 days travel on the Island of Ireland (Eire + NI) but get unlimited free use in Scotland, Wales and England. 

Thank you for providing further clarity. It's important to note that the Interrail Global Pass has specific rules regarding travel days within one's country of residence. This distinction helps travelers understand the limitations and benefits of using the Interrail Global Pass based on their residence. It's always advisable to carefully review the terms and conditions of the pass before planning your travel.