Its kick started the two year process of the UK's divorce from the European Union.
European Council President Donald Tusk confirmed he has received the letter.
Moments after it was delivered Former Scottish First Minister Alex Salmond suggested now was not the right time to do it.
In the Dáil there were fresh calls for Northern Ireland to be given a 'special status' in the EU and the Brexit talks.
Minister Paschal Donohoe says the government has fully expressed the unique status of the entire island.
Fianna Fáil says there's no clarity on what the government would deem a “success” in the Brexit talks.
It says there's been too much of a focus on the diplomacy relating to Northern Ireland and not enough on enterprise.
