US President Joe Biden has spent his first day in office signing orders to undo several policies of his predecessor.
His inaugural address on Wednesday talked about unity in a country riven with division.
He then went on to sign 15 executive orders and two directives - all aimed at swiftly undoing Donald Trump's work.
This included moves to re-join the Paris Climate accord, a national mask-wearing mandate and reversing the Muslim travel ban.
The US will also seek re-admission to the World Health Organisation (WHO).
Speaking after he was sworn in on Wednesday, President Biden told the gathered crowd that: "Democracy has prevailed."
He said his presidency would be about uniting the nation and he urged "every American to join me in this cause."
"Few people in our nation's history have been more challenged or found a time more challenging or difficult than the time we're in now.
"To overcome these challenges, to restore the soul and secure the future of America, requires so much more than words and requires the most elusive of all things in a democracy; unity."
US President Joe Biden and First Lady Dr Jill Biden arriving at the US Capitol in Washington DC. Picture by: Jim Lo Scalzo/AP/Press Association ImagesHis vice-president has already made history - Kamala Harris is the first woman and the first black person and person of south-Asian descent to take on the role.
Taoiseach Micheál Martin congratulated Mr Biden on taking office, saying: "The inauguration of a new US president is a moment of hope and renewal in US political life.
"As he takes the oath-of-office, I know that President Biden will feel the weight of history - the presence of his Irish ancestors who left Mayo and Louth in famine times in search of life and hope.
"He will remember their descendants' hard struggle as they made their way in and their contribution to the United States. It is the story of Irish-America.
"I hope he will also be conscious of the great pride we in Ireland take in his immense achievement. He is one of us, part of our global family."
Joe Biden's cousins Joe Blewitt with daughters Emily (left) and Lauren (right) pose beside his mural in Ballina, Co Mayo. President Biden's great-great grandfather, Patrick Blewitt, was born in Ballina in 1832 and left for the US in 1850. Picture by: Peter Morrison/AP/Press Association Images"I believe Ireland can be a bridge-builder between the United States and the European Union as we all share common values in securing democracy, peace and economic development", he added.
He also paid tribute to Vice-President Harris, who "has created history today, and who will help form a diverse and formidable administration."
President Michael D Higgins has written to President Biden offering him "heartfelt congratulations".
"There can be no greater honour than to be given the trust of one's fellow citizens to lead," he wrote.
"There can be no greater honour than to be given the trust of one's fellow citizens to lead."
President Michael D. Higgins has written to President Joe Biden, offering his "heartfelt congratulations" as @JoeBiden assumes the office of President of the United States. pic.twitter.com/grdf6hBVYN— President of Ireland (@PresidentIRL) January 20, 2021
"The US has been a true friend to Ireland in so many ways. Your own friendship and support for so many years has been invaluable.
"Ireland, of course, has made its most valuable contribution to your great land by providing so many of our daughters and sons.
"The descendants of some turned out to rather fine Presidents!"
Fifth cousins of US President Joe Biden, Councillor Andrea McKevitt (right) and her sister Ciara, at their home on the Cooley Peninsula in Co Louth, mark his inauguration. Picture by: Brian Lawless/PA Wire/PA ImagesHe added that he hoped to welcome President Biden and Dr Jill Biden "to Ireland once again, as soon as circumstances permit".
While European Commission President Ursula von der Leyen said: "Thank you for the inspiring inaugural address and for the offer to cooperate.
"Europe is ready for a fresh start."
Congratulations @JoeBiden on being sworn in as President of the United States and @KamalaHarris – the first woman Vice-President of the US!
Thank you for the inspiring inaugural address and for the offer to cooperate.
Europe is ready for a fresh start.#InaugurationDay pic.twitter.com/Re7vaoUlHS— Ursula von der Leyen (@vonderleyen) January 20, 2021
"This new dawn in America is the moment we have been waiting for, so long.
"Europe is ready for a new start with our oldest and most trusted partner.
"And this is why the commission recently has adopted a new, forward looking transatlantic agenda.
"And it focuses on where we can together join forces to push for global change – based on our common values, interests and global influence."
"From climate change to health, from digitalisation to democracy - these are global challenges that need renewed and improved global cooperation.
"And the European Union and the United States must lead from the front and bring an alliance of like-minded partners with us.
"There is of course no greater need for global cooperation in the short term than in tackling the pandemic, which has been so devastating on both sides of the Atlantic."
"This day brings good news: The United States are back. And Europe stands ready. To reconnect with an old and trusted partner, to breathe new life in our cherished alliance", she added.
Additional reporting: IRN
