You will also have been paying SSN for 8 years, I believe in the US to be eligible for Social Security in retirement you need 10 years of contributions, that is something to look into.
I did a 401k rollover to a rollover IRA directly before leaving the US and thus did not have to pay early withdrawl charges. Do you get penalised for converting to a Roth? My idea is to let it grow until retirement age and then withdraw it all and bring it back to Ireland.
You need to find a specialist to find out whether this would fall under a tax treaty between Ireland and US and avoid double taxation.
The issue I see is that you are doing the backdoor roth as a US Tax resident but would be accessing the funds in future as an Irish tax resident and thus subject to our tax rules.