It's the political rivalry that has dominated Irish politics since independence. Every government over the last century has been led by one party or the other - and now, for the first time, they are set to govern together.
As many people have pointed out, this may not make that much difference to policy - they are, after all, both broadly centre-right parties. But it will certainly make a huge difference to politics, changing the rules of the electoral game for ever.
Both Fine Gael and Fianna Fáil trace their roots back to the united Sinn Féin party, which swept the boards in the 1918 general election and led Irish nationalism during the War of Independence. The party split over the Treaty, the agreement reached in December 1921 between British and Irish negotiators over the future status of the independent Irish state.
Despite a common assumption, the split had very little to do with Partition - a subject barely mentioned during the heated Treaty Debates - but was about the status of the 26-county State. Would it be a Republic, or a Dominion within the British Commonwealth?
Supporters of the Treaty, led by Michael Collins and Arthur Griffith, believed that Dominion status would allow Ireland to gradually assert its sovereignty until it became fully independent. Opponents of the Treaty - led somewhat reluctantly by Éamon de Valera - thought Dominion status was a betrayal of the sacrifices of the Rising and the War of Independence, and would trap Ireland forever under British rule.
The increasingly bitter row led to the lamentable Civil War, for which both sides (along with the British government) share the blame. Thousands died, and atrocities were committed by both sides, further inflaming the bitterness which was passed down through the generations.
Collins and Griffith both died during the Civil War, and WT Cosgrave became leader of Cumann na nGaedheal, which governed the new Irish Free State for its first decade, securely establishing its institutions, though it also became seen as conservative, penny-pinching, and more pro-British than Collins ever would have been.
De Valera, after Civil War defeat and imprisonment, founded Fianna Fáil, which attracted many anti-Treatyites. He took power in 1932, introducing social reforms and protection for Irish industry, breaking further away from British control, and remaining neutral in the Second World War, the ultimate expression of sovereignty.
But while the origins of both parties, and the split between them, lay in the Civil War, it is slightly misleading to call them "the Civil War parties", because both were formed in a conscious attempt to transcend the limitations of that split.
De Valera left anti-Treaty Sinn Féin and formed Fianna Fáil in 1926 precisely because he wanted to appeal to those who were neutral in the Civil War, or even supported the Treaty. After all, while Sinn Féin had done surprisingly well in the 1923 general election, it had still lost. And de Valera liked to win. So he explicitly stated that Fianna Fáil would be a broad church which would try to recreate the united movement of 1918-21.
Fine Gael was also an attempt at broadening support. While Cumann na nGaedheal had undoubted successes during its ten years in power, it had also become deeply unpopular. After Fianna Fáil took power in 1932, and then won an overall majority in a snap election in 1933, Cosgrave and his colleagues realised the need to rebrand. They also genuinely if wrongly worried that de Valera would ban opposition.
It decided on a three-way merger to create a new party. Joining Cumann na nGaedheal in Fine Gael was the Centre Party, which represented former supporters of the constitutional Home Rule Party, who had largely remained neutral in the Civil War. The third part of Fine Gael was more controversial – the quasi-fascist Blueshirts, whose uniforms and Nazi-style salutes echoed contemporary trends in Europe. The Blueshirt leader, Eoin O'Duffy, actually became the first leader of Fine Gael, an arrangement which didn't last long due to his utter unsuitability for political office, and Cosgrave returned to the leadership.
But while the two parties had attempted to transcend the Civil War split, the issue which caused the conflict, the State's constitutional status, remained - at least until 1937, when de Valera's Constitution made Ireland a Republic in all but name. The question was finally put to rest when a Fine Gael-led government declared a Republic and formally left the British Commonwealth in 1949.
With the resolution of this issue, there was no longer any political reason for the so-called Civil War split - and yet the enmity and rivalry between Fianna Fáil and Fine Gael endured for another 70 years. Why?
I contributed to an Explained by Prime Time last year about what real difference there is between the two. Spoiler alert: The answer was, not much.
In fact, back in 1999 political scientists Michael Gallagher and Michael Marsh asked members of Fine Gael what separated their party from Fianna Fáil. No less than 39% of them said there was no real policy difference at all between the old rivals.
Partly, the very fact of the rivalry has helped perpetuate it. Both Leo Vardkar and Simon Coveney in recent days have compared the relationship between Fine Gael and Fianna Fáil to that of GAA teams from neighbouring parishes, each defining themselves by how well they perform against the other. Fianna Fáil Taoiseach Seán Lemass once said something similar. Asked to explain the difference between his party and its traditional rival, Fine Gael. The answer, he said, was simple: "We're in, and they're out."
There are, though, very real differences of perception – how they see themselves, and how they see each other. Fine Gael sees Fianna Fáil as slightly untrustworthy; Fianna Fáil sees Fine Gael as the party of the privileged elite, out of touch with the plain people of Ireland.
Fine Gael would see itself as the party most loyal to the State (which it always points out was founded by the party's forerunners), and most strongly supportive of law and order. More responsible, more honest in its own estimation – and probably less fun, according to some focus group research on public attitudes to the two parties.
Fianna Fáil members insisted it wasn't a party, but a national movement, representing all classes and sectors of society. So while it claimed to be the real labour party, and consistently won huge working class support, it was also the party of the businessman and the entrepreneur; it won the rural vote, and also the vote in the towns and the cities. It was better at elections, better at politics, and – it claimed - better at government than Fine Gael. Until the economic crash destroyed its reputation for competence, and left it trailing its traditional rival.
So if there's not that much separating Fine Gael and Fianna Fáil, as they are both centre-right parties with very similar views on many issues, why all the talk about their proposed new government being an historic watershed?
Perhaps because Irish politics has, for a century, been structured around the rivalry between Fianna Fáil and Fine Gael. Now, they could be in government together, presenting a gilt-edged opportunity to their opponents on the left to recast political competition. While Sinn Féin will complain loudly about the new coalition, it will also know that it will be the main beneficiary if things come unstuck.
To return to where we came in, the programme for government comments, with some understatement, that "Ireland is facing a very complex period, as we plan commemorations over the next few years".
Those complex centenaries include the signing of the Treaty, the split in Sinn Féin, the Civil War, and the foundation of the Free State on 8 December 2022. Exactly one week later, on 15 December, the Fianna Fáil Taoiseach is due to hand over his office to his coalition partner, the leader of Fine Gael.
History isn't everything, but it is something, and its weight will be felt over the new coalition's term.