From the beginning, Mr Trump has created nothing but headaches for Mr Ryan, the Speaker of the House of Representatives and the nation’s top Republican, as he tries to marshal his fractured party behind the bombastic billionaire.
Mr Ryan has always been reluctant to support Mr Trump, and openly criticised the candidate’s proposed ban on Muslims entering the US. He’s also called other remarks by the candidate “an example of textbook racism”.
Nevertheless, for the sake of party unity, Mr Ryan eventually decided to back Mr Trump, whom he essentially sees as a Trojan Horse who would push the Republican congressional agenda once in the White House.
However, Mr Ryan’s stance has diminished his standing among some in the party.
At a CNN political event last week, for example, a Republican voter rounded on him, questioning how he could “morally support” a candidate who is “openly racist and has made Islamophobic statements”.
If things go badly for Mr Trump in his battle against Democrat Hillary Clinton, Mr Ryan may end up with neither a Republican president nor a Republican Congress because polls show that not alone could Mr Trump lose the election to Ms Clinton, but he could also doom the party’s candidates in the House and the Senate elections.
Then the blame game will start and Mr Ryan will be slap bang in the middle of it, potentially fighting for his political life.
At 46, however, he should have time to regroup and, in the process, boost his own presidential ambitions.
Indeed, many in the party have made no secret of the fact that they would have much preferred him as their presidential candidate this year rather than Mr Trump.
He is a Tea Party favourite and has all the right conservative credentials.
He is also a strong media performer in pushing the Republican agenda. He is a harsh critic of some of America’s anti-poverty programmes.
In 2012, when he was Mitt Romney vice presidential running mate, he said that the network of programmes for the American poor made people not want to work.
Then there was the story he told of a boy who did not want his free school lunch because it left him with “a full stomach and an empty soul.”
The Washington Post said the story was untrue. It had apparently been lifted from a memoir, The Invisible Thread, about a friendship between a middle-aged white woman and a black boy she befriended on the streets of New York.
The remark provoked a scathing attack on Mr Ryan by New York Times columnist Timothy Egan, who said Ryan’s Irish roots and his awareness of the Famine meant he should know better.
“But he hasn’t learned anything,” said Mr Egan. “His great-great-grandfather had fled to America. But the Republican congressman was wagging his finger at the famished.”
As a youth, however, Mr Ryan himself benefited from America’s fragile social security net. When he was 16 he found his 55-year- old father lying dead in bed after a heart attack.
After his father’s death, Mr Ryan received social security benefits until his 18th birthday, which were saved for his college education. He also helped to care for his grandmother, who had Alzheimer’s, while his mother commuted to college.
Mr Ryan’s Irish roots go back to Graiguenamanagh in Co Kilkenny, which the family has visited. He has been honoured by the American Ireland Fund and is seen as supportive of immigration reform that could help thousands of Irish illegals.