Joe Biden was born into a proud Irish American Catholic family in the blue-collar city of Scranton Pennsylvania - a state his ancestors settled and met in.
While the president has pronounced Ireland "part of his soul", it is also part of his DNA.
"My Grandpa Finnegan would also say… 'Remember, Joey, the best drop of blood in you is Irish,' he quipped during an address at the Windsor Bar in Dundalk on Wednesday evening.
But this is not just about the wise words of Grandpa Finnegan, the facts and historical figures are there to back it up.
His strongest ties appear to be with Mayo but there are also links to Louth, Galway and Donegal.
Ten of his 16 great-great-grandparents were from the Emerald Isle.
While nine of them were born in Ireland, the tenth was the daughter of Galway immigrants. Mary Ward was born en route to the United States and would, years later, marry a Galway man.
"To paraphrase the Duke of Wellington - just because you're born in a stable, it doesn't make you a horse. So, we claim her as being Irish," said Fiona Fitzsimons, Director of the Irish Family History Centre, who along with Helen Moss were commissioned by Joe Biden when he was vice president to delve deeper into his family's roots.
Fiona Fitzsimons said the president stands out not just because of his strong Irish links but because of the "extraordinary" fact that all his ancestors are considered "famine Irish" – meaning they arrived in the US between 1848 and 1861.
"Normally when you get that deep rooted Irishness, you might find somebody coming in, in the early 20th century… But this is what's extraordinary … is that all of these are famine Irish," said Ms Fitzsimons.
Four of his ten great-grandparents are from Mayo - the Stantons, the Arthurs, the Basquilles and (probably the most well-known) the Blewitts.
His third great-grandfather Edward Blewitt took the five-week journey across the ocean to the United States and set up his family in Pennsylvania.
Before he took the 'coffin ship' across the Atlantic to begin a new life in America, Mr Blewitt worked for the ordnance survey office - walking about the land and helping making maps in the mid-1800s.
According to Fiona Fitzsimons, a fascinating part of the Blewitt story is that it appears Edward and his brother James were educated in a hedge school.
"Yet somehow, [the brothers] picked up enough higher maths to be able to parlay that into a surveyors job," said Fiona.
She said Edward Blewitt's drive and "entrepreneurial" side would lead to a successful life for him and his family in the United States. His grandson would go on to become a very early Irish Catholic member of the senate in Pennsylvania.
Decades later his grandson's daughter, Geraldine C Blewitt would marry Ambrose J Finnegan - another second-generation Irish person. This branch of the family tree stems from Co Louth.
The Finnegans and the Kearneys, his third-great grandparents, lived right out by the sea on the Cooley Peninsula.
"The Kearneys being very industrious, they collected the seaweed off the beach. Part of their agreement with the landlord was that they could have some of the seaweed… but on the side they were selling the off product of the seaweed to the other farmers in the area," said Kayleigh Bealin, Research Manager with the Irish Family History Centre.
Not surprisingly the side hustle, when discovered, left the landlord unimpressed.
Owen Finnegan and his family left for the United States in 1849, opening a business as a shoemaker in Seneca Falls New York.
His son James would be the one to relocate to Pennsylvania along with his wife Catherine Roche and their six children - one them Ambrose J Finnegan, or Joe Biden's grandfather, who would marry Geraldine C Blewitt.
While the focus of this presidential visit to Ireland has been about Mr Biden's lineage in Co Mayo and Co Louth, there are more Irish branches to his family tree.
Joe Biden also has connections to Donegal. His great-great grandmother, Catherine Scanlon was from Co Donegal but grew up in the United States after her father, Anthony Scanlon brought his family to the US around 1848 when she was around 10 years old.
"We believe Anthony Scanlon was originally from Co Mayo… he was a coastguard," said Ms Fitzsimons.
In the 1800s, coastguards were an emerging profession and were first recruited as revenue enforcers – hired to prevent smuggling of wine, brandy, whiskey, and tea. Later in the 1840s/1850s they were moved into saving lives.
"Anthony Scanlon was posted at different times around the coast, and his children appear to have been born in Donegal. We think probably Ballyshannon. We have Anthony Scanlon there in the 1830s, but unfortunately the parish registers don't start early enough for us to find baptisms," said Fiona Fitzsimons.
From stories passed down through generations and more connections revealed over time, Ireland has always been a huge part of Joe Biden's life.
Many of the president's ancestors left an Ireland ravaged by famine - a tough reality that Joe Biden laments as he makes this historic visit.
"It feels like home. I know why my ancestors and many of your relatives left during the famine and - but, you know, when you're here, you wonder why anyone would ever want to leave."
