United Kingdom - Passport & Nationality - Irish Citizenship by Double Descent (GP born in Ireland)
If you were born outside Ireland to a mother or father who were themselves born outside Ireland and if any of your grandparents was born in Ireland, then you are entitled to register as an Irish citizen. However before you can claim Irish citizenship, you must have your birth registered in the Irish Foreign Births Register.
The Irish citizenship of successive generations may be maintained by each generation ensuring their registration in the Foreign Births Register before the birth of the next generation. Children born to that generation before its date of entry in the Register are not entitled to citizenship. Unless at least one parent or grandparent was an Irish citizen at the time of your birth, you cannot claim Irish citizenship on the basis of extended previous ancestry or wider family ties such as the Irish citizenship of cousins aunts, uncles etc.
Irish nationality can ONLY pass down by descent if the birth is legitimate.
Finally, an Irish passport will allow you to live and work in the UK. This right pre-dates the UK's entry into the EU, and is therefore not affected by the UK's recent exit from the UK.