Listening to Mary Lou McDonald over the weekend, it was difficult to avoid the conclusion that not being a mandatory component in the Irish Government had come as a bit of a shock to Sinn Fein: "It was absolutely wrong and flew in the face of the democratic choice of the people to deliberately exclude Sinn Fein."
Fair enough, Sinn Fein had a very good election in February - much, much better than even it expected - but McDonald wasn't able to build a majority during a series of negotiations with other parties and independent TDs.
That failure to build her own coalition left the door open for others to try. And that's precisely what Fianna Fail, Fine Gael and the Greens did.
Mind you, until the very last moment, when the Greens declared in favour of the deal, Sinn Fein still hoped it could be included in some sort of coalition.
Speaking to the Irish Independent on Friday about what his party would do if the Greens didn't agree to back the deal, Eoin O Broin said: "So, in the first instance, what Sinn Fein would try and do is engage with the kind of pro-change, progressive parties to see if we can take the programme for government draft that's there and turn it into a genuinely pro-change programme for government. And, of course, I'd be willing to sit down with Fianna Fail and say, 'Would you be willing to support that kind of agenda?'"
Sinn Fein has been spoiled when it comes to its experience of government. In Northern Ireland, there can't be an Executive unless it chooses to be in it. It can block any policy it doesn't like. It can override the views of a majority of the other parties in the Executive and ignore the votes of the majority of MLAs in the Assembly.
It can, if and when it so desires, bring down the entire structures of government. It can insist that every decision is built around a quid pro quo process. In other words, if it doesn't get what it wants, it can simply lift the ball, walk off the pitch and lock down the grounds.
That's not how it works in other places. All that is required for a government in the south is a majority of TDs to support it.
How that majority is made up is neither here nor there. However unusual the make-up of the coalition may be is neither here nor there. There is no requirement for the party which did best at the election to be included.
And no amount of Sinn Fein tweeting photographs of Mary Lou McDonald under the mantra, The People's Taoiseach, is going to change any of these realities.
Basically, it's a case of suck it up and work out what to do next.
Sinn Fein is not afraid of Opposition. McDonald has been sounding out her 36 TDs about what roles they might have on the shadow frontbench and she has already talked about building the "most coherent and effective Opposition in history".
But Opposition, particularly the role of "official Opposition", was never her preferred choice. That's because you can't walk out of Opposition in the same way you can walk out of government. And Sinn Fein has a history of walking out of government in Northern Ireland when circumstances demand, or the grassroots dictate.
A role in a coalition government (and I think it was always unlikely that she would have been Taoiseach this time around) would have left open the walk-out option if the party detected a shift against it in the polls, or found itself facing the prospect of some difficult, unpopular decisions further down the line.
More important, though, a role in a coalition would probably have involved a promise that the government committed to a border poll and a substantial game-plan for unity. The Fianna Fail/Fine Gael/Green coalition's proposals on "A Shared Island" come nowhere close to Sinn Fein's hopes of just a few months ago. And it will be keenly aware that five years is a long time for their "unity project" to be gathering dust on the Martin/Varadkar back-burner.
While Opposition is clearly not the party's first choice, for now it is the only choice. And, in some ways, that is no bad thing. It has a lot of new blood with little experience at this level of politics and Opposition is the perfect testing and learning ground for those TDs.
Some of the greatest careers in politics have been honed and forged during early years in Opposition, particularly, like now, with an unlikely coalition government handling the aftermath of a crisis.
And the coalition itself may not prove as stable as it will need to be. Indeed, there are no guarantees that the government will even last a full term.
But Opposition also comes with enormous risks and challenges. Sinn Fein will have to prove that it can land significant blows on the government; but those blows will depend on it being able to pick apart government policy and setting out a clear, popular alternative of its own.
And while you'll never see a hint of it in even the smallest of small print in the programme for government, you can bet your bottom euro that Fianna Fail and Fine Gael will have a group of people within a Press office whose only job is to deconstruct and destroy Sinn Fein's economic figures and broader policies.
Martin and Varadkar have a joint vested interest in keeping Sinn Fein out of government and they will throw everything at the party, including a lot of money at new voters who voted for the party in February.
What Sinn Fein really needs to get its head around is the fact that politics in the Dail is not the same as politics in the Assembly. The nature of the power it has in both institutions is entirely different.
It cannot make or break the coalition in Dublin the way it can make and break a coalition in Belfast. It may also discover that increasing numbers of the southern electorate - faced with a pressing array of post-lockdown problems - will have no interest in Sinn Fein's prioritising a unity agenda.
At times like this, the realities of the here-and-now will usually trump constitutional aspirations. Which is why, I think, the new coalition is so woolly on the all-island stuff.
This is a huge moment for Sinn Fein. The role of official Opposition gives it the chance to present itself as the alternative government, as voters see how it responds to challenges it hasn't really had before.
It won't be easy. Sinn Fein has sounded a bit needy and self-important over the past few weeks and absurdly whiny - as well as wrong - about being "kept out" of government (which it wasn't).
It needs to get over itself and rise to new and unexpected possibilities.