Northern Ireland elects Irish nationalist First Minister Michelle O'Neill

, do you think this signals a broader shift in the electorate towards supporting Irish nationalism (is this functionally the same as supporting reunification?), or is this more a backlash against DUP? That is, do you think among the average voter support has grown for reunification?

Yes, yes, and yes. There has been a steady swing towards unification with the ROI (Republic of Ireland, the other half of the island), and the Tories in the UK in supporting Brexit wrote off NI.

Supporting both required simultaneously having a land border with the EU (meaning customs checks to prevent products that aren't up to snuff from getting into the EU, or evading taxes), and also not having any kind of border with Northern Ireland, (because the right to travel freely between them was a huge part of the peace deal that, essentially, treats everyone in NI as both UK and Irish citizens at the same time). Violating that deal would absolutely mean a return to terrorism, and the first target is going to be that border. Caught between the popular vote of the day and lasting peace in Northern Ireland, the Tories made the obvious choice and sacrificed ties to NI, suggesting it could be an EU zone inside the UK for trading purposes and putting the customs border in the sea between Britain and the island of Ireland instead. This completely contradicted their promise that Northern Ireland was 100% part the UK, especially as the DUP also supported Brexit and wanted to leave too.

When this became clear to the DUP, who to be clear are a bunch of evangelical Protestants (see their stances on LGBT rights (see "Save Ulster from Sodomy" campaign), abortion, Creationism in schools, all things the ROI has gotten rid of), they took the only action available to them to avoid losing more power, namely stopping government from working entirely until circumstances were more favourable.
As you can see, it didn't work at all.

We are probably still a long way off Irish unification (if it happens at all and not for the reason you might think) – it's hard to see why the Republic would want NI; I would not be surprised if at some point there is a referendum where NI votes to unite with the Republic and the Republic votes to reject unification.

The reasons are primarily economic – Ireland would have to spend a lot of money on subsidising NI (the UK government spends something like ~10 billion per year, roughly 10% of Irish government spending). There's also the potential for things to get out of hand when it comes to terrorism – Ireland lacks the same kind of security apparatus the British state has (for context UK government spending on intelligence is more than that of Germany and France combined and UK defence spending is 50 times larger than Ireland's). Unionist terrorism may not end up being as well organised as the IRA but given the IRA's success through terrorism it's not that unlikely that unionist paramilitary groups might start organising again.

See this poll for example which found that

two-thirds of people in the Republic support unification of their island, according to the poll published Saturday in the Irish Independent... Fifty-four percent of Irish Republic voters would reject unity if it hikes their tax bills, according to the survey, which was conducted by the polling firm Kantar... Only one in eight would vote for unity if the handover required the Republic to take on Britain’s full costs of subsidizing Northern Ireland.

Given that it's inevitable that the Republic would have to (at least initially) provide the same level of financial support to NI as Britain does currently, it will be interesting to see what the outcome is.