I grew up and worked in Northern Ireland until I was 25, I have lived in the Republic for the past 8 years but visit home regularly, am I covered under the NHS when I visit?
The E111 would automatically cover you in respect to travel out of State if you needed medical/hospital care while out of State.
If you also have an address in NI ie. parents - you qualify for the NHS AFAIK.
There is EU Reg 1408/71 which entitles you to ambulatory services and to be reimbursed up to the same level of benefits as if you accessed them in S. Ireland. You dont need prior authorisation for ambulatory care. This includes dental and optical benefits - although the current level of benefit entitlements has been pulled back by the DSFA.
If you are planning to obtain planned hospitalised care you need to obtain prior authorisation from the HSE to travel and receive planned hospitalised care out of State. Travel for hospitalised care is allowed in circumstances where the treatment is not available in S. Ireland or within a timeframe given your health needs. The treating consultant or hospital admin should know the process to be followed on travelling to another EU member State.