Irish Citizens who already live in the UK, or who may move to the UK in the future, will not be impacted by the UK's withdrawal from the European Union. This is because Irish citizens enjoy a right to live and work in the UK which is not reliant on the UK's membership of the EU, rather it is based on the Common Travel Area.
Irish citizens will continue to have a right to live and work in the UK beyond the end of the Brexit Transition period as a result of the Common Travel Area arrangements in place.
However, non-EEA/Swiss family members of Irish citizens may need to apply under the EU Settlement Scheme to continue living in the UK beyond the end of the Brexit Transition period. Further details on the EU Settlement Scheme and eligibility can be found here.