I wonder why nobody seems to have ever taken the condominium solution seriously?

In a condominium, Northern Ireland would become a joint territory of both the UK and the Republic of Ireland. It would be institutionally complex, but I can't see why it couldn't be made to work in practice, if the constitutional decision was made to implement it.

It would be similar in some ways to Andorra – just as Andorra has two joint heads of state (the Catholic Bishop of Urgell in Spain, and the President of France), Northern Ireland would have two joint heads of state (the President of Ireland and the King/Queen of the UK). I doubt they'd call them "Co-Princes" though, as Andorra does–too mediaeval-sounding. And, unlike Andorra, I'm not talking about Northern Ireland being an independent sovereign state – it would still be a subnational unit, just a subnational unit belonging to two sovereign states simultaneously.

While such arrangements are quite rare, they are allowed under international law, and have existed in the past. A notable example is Vanuatu, which prior to independence in 1980, was a joint British-French territory (New Hebrides). However, New Hebrides is in some ways a bad example - pre-independence it had two separate governments, one British and one French, with immensely inefficient duplication–two separate police forces, two separate prison systems; even two separate immigration departments–if the British one refused you a visa, you could always have another go with the French one. Initially, the only shared institution was the Joint Court, with three judges–one British, one French, and a third neutral judge to act as a tie-breaker, appointed by the King of Spain. In the later years, they introduced a third joint British-French government, and transferred some powers to it-but the separate British and French governments still remained, and continued their inefficient duplication right up to independence.

If there were ever a Northern Ireland condominium, I would expect something far more streamlined, in which Ireland and the UK appoint a joint administration in Belfast to run non-devolved matters, to which the current powers of the UK ministers would be transferred, while the existing devolved institutions would continue to exist under that joint administration's supervision. The Irish and UK national Parliaments would each separately have the power to disallow secondary legislation made by the joint administration, and primary legislation in non-devolved matters would be proposed by the joint administration, and then only enter into force once both national Parliaments had separately (and identically) enacted it.

Another major difference is that New Hebrides was a colonial situation, in which the indigenous population were denied political power – and, as was common in European colonialism, even the citizens of the colonial powers found themselves denied in the colonies political rights they would have had in they'd remained in the mother country. I think a solution for a Northern Ireland condominium could be for Northern Irish MPs to be members of both the Dáil Éireann and the House of Commons simultaneously. They could also play a special role in providing democratic accountability to the joint administration, through a "Northern Ireland Grand Committee" (and possibly subcommittees thereof) which would simultaneously be a committee of both national Parliaments (all its members being simultaneously members of both). And the final court of appeal for Northern Ireland would have an equal number of judges appointed by each national Supreme Court, from among their own members – maybe with (like the Joint Court of New Hebrides) a Chief Justice being appointed from a third country – these days, rather than turning to a third country's monarchy to appoint a neutral arbiter, you might give that power to the Secretary-General of an international organisation, such as the Permanent Court of Arbitration, or the Council of Europe.

Probably never going to happen – I expect the status quo to endure for a few more decades, followed by eventual Irish reunification – but a condominium is an interesting idea to ponder.