The last pillars of political certainty came crashing down a week ago in the referendums on the Belfast Agreement. Already, over the last 30 years, a number of others had fallen: the notion that the essence of Irishness was to be found in a potent concoction of faith and fatherland. The industrial nexus of ships, steel and skills that defined a Protestant place in the world. The intertwining of church and state through crozier and Constitution or through Queen and Empire. The shared culture of deference, obedience and gratitude for small mercies. Each has been undermined by the turbulent course of events. Now, the biggest ones of all have crumbled. Irish nationalism and Irish unionism - in the sense that those words have been used for most of this century - are dead.
Nationalism has been so radically transformed as to be unrecognisable. The Irish nation, in the new Article 2 of the Constitution, is something to which people have a right, but not a duty, to belong. The whole way of thinking that came out of 19th century nationalism, in which the nation had sacred claims on its people, has been reversed. Now, it is the people who have claims on the nation. Those born on the island have an "entitlement and birthright" to be "part of the Irish nation". But the nation is not entitled to demand their allegiance. The right to belong clearly contains a right not to belong.
By extension, the traditional demand for a united Ireland has been turned into something utterly different from what either Collins or de Valera ever imagined. The new Article 3, by writing in the principle of consent, effectively recognises the legitimacy of Northern Ireland's present place in the United Kingdom. At the same time, the eventual united Ireland that it envisages is not what it used to be either. The aspiration now is not towards a monolithic political structure, but to a unity of people "in all the diversity of their identities and traditions".
By definition, a united Ireland that might eventually meet this aspiration would be a complex set of political arrangements, not a centralised nation state in the old sense. Whatever else we want to call that aspiration, nationalism is no longer the right word for it.
On the surface, the death of traditional unionism is less obvious. Unionists can say with a great deal of justification that their right to remain within the UK has been accepted and secured.
But the meaning of that right has been altered forever. From now on it can be exercised only in the context of a much wider set of relationships with Northern Catholics, with the Republic, and with an increasingly decentralised Britain.
Even more profoundly, unionism is clearly no longer the political ideology of a unified Protestant community. It is undoubtedly true that the vast majority of Protestants will continue to regard themselves as British, to oppose a united Ireland, and to vote for parties that do likewise. But the cleavage between David Trimble and Ian Paisley is now so fundamental that it makes little sense to talk of them as belonging to the same broad political movement. And the essence of unionism has long been the idea not just of uniting Northern Ireland to Britain but of uniting Protestants to each other.
The division between Yes and No forces within unionism isn't just a matter of tactics. It has deep roots in religious and cultural fragmentation. Catholics, who are used to thinking of "the church", nearly always forget that for the Protestant population there is a multiplicity of church affiliations - Church of Ireland, Methodist, Presbyterian, Reformed Presbyterian, Free Presbyterian, Evangelical Presbyterian, Independent Evangelical, Baptist, Brethren, Independent Charismatic Fellowship, Independent Pentecostal, Elim, Moravian, Church of God, Assemblies of God and, of course, atheists and agnostics who were brought up in one or other of those churches and who remain part of a broad Protestant culture.
In some respects these religious divisions are as strong as those between Protestants and Catholics. It is striking, for instance, that in a study of attitudes among Belfast churchgoers, published as Them and Us? last year by the Institute of Irish Studies at Queen's University, mainstream Protestants were more willing to attend joint church services with Catholics than with fundamentalist Protestants. Less than half of Church of Ireland, mainstream Presbyterian and Methodist churchgoers said they found acceptable the idea of having joint services with Ian Paisley's Free Presbyterians. As the authors of the study suggest, "the fractures within Protestantism on ecclesiastical matters run deep, deeper perhaps in some cases than the chasm of conventional Protestant-Catholic discourse." When people talk about "the religious divide" in Northern Ireland, they are almost never referring to this split within Protestantism. But the divide is there and it relates to other cleavages between city and country, and between rich and poor. One of the many ironies of the IRA's terror campaign is that it almost certainly pushed Protestants together and helped to conceal the fissures within their culture. With the waning of violence, those cracks are appearing again. The more obvious they become, the clearer it will be that unionism as a monolithic, unifying ideology is dead.
The ruins of all of these ideologies are a fine sight. Given the cruelty and waste they have generated, no one should be sorry to see them go. But we should worry about what will take their place. And for that, too, last Friday offered some pointers. Amidst all the joy, there was also an immensely depressing fact - the failure of close to half of the Republic's electorate to vote at all.
Maybe it's because I flew a few thousand miles to vote myself. Maybe it's because I left behind in America many, many Irish citizens who desperately wanted to vote but didn't have the means or the opportunity to get home.
But I was really shocked to find on Saturday that over 1.2 million members of the Republic's electorate couldn't be bothered to give five minutes of their day to the most important Irish political event in most of our lifetimes. The only explanations are utter alienation and sheer, mindlessly selfish apathy. That, perhaps, is the dark side of the death of ideologies. Too strong a political conflict is, as we now know too well, deathly. But when there is no passion left, there is a different kind of death - the demise of politics as a means to secure social justice and human dignity. If the terrible inertia of civic culture, exemplified by the low turnout last week, is the best the Republic has to offer, nothing very enduring will be built on the ruins of the old ideologies. Unless newly fashionable words like "equality" and "respect" are given real economic and social substance, we will be left with nothing to stand on but the rough fragments of demolished ideologies.