A person who lives in one country and works in another, returning to the country they live in at least once a week is aa cross border worker (also called a frontier worker). For example, a person who lives in Ireland and travels to work in Northern Ireland is a cross border worker.
After Brexit, the UK introduced rules that affect EU citizens who work in Northern Ireland (and the rest of the UK). However, these rules do not apply to Irish citizens.Irish and British citizens have the right to live and work in both Ireland and the UK because both countries are part of the Common Travel Area. Common Travel Area rights only apply to Irish and UK citizens, and not to citizens of other countries who live in Ireland or the UK.
So, if you are an Irish citizen you do not need a Frontier Worker Permit.
However, if you are an EU citizen who was a cross border worker before 1 January 2021, you must apply for a Frontier Worker Permit to continue working there after 30 June 2021. You can apply for the permit online. There is no charge for a Frontier Worker Permit.If you became a cross border worker after 31 December 2020, you are not eligible for the Frontier Worker Permit. You must apply for permission to work in the UK as part of the UK’s Points Based Immigration System.
EU citizens who live in Northern Ireland and work in Ireland do not need to apply for permission to work in Ireland. You must apply for UK residence through the EU Settlement Scheme (if you were living in the UK before the end of 2020) or the Points Based Immigration Scheme (if you have moved there since the beginning of 2021).
You can find out more about cross border workers on citizensinformation.ie.