It is ironic that Brexiteers who yearn for British independence from the European Union are often fervently against any nation's independence from the United Kingdom. Yet Brexit would have big repercussions for Scotland and Northern Ireland, and to some extent Wales.

In all three, the debate has been more subdued than in England, perhaps because majorities of voters look likely to back the Remain side (unlike the 1975 referendum, when they were all less keen than England on Europe). But this means that Brexit, were it to happen, would be imposed by English voters against the wishes of many along the Celtic fringe.

It would certainly rile the Scots, who see Brussels as a sort of alternative power center to London. A recent debate in the Scottish parliament found all five main parties there backing Remain. Opinion polls suggest that as many as 75 percent of Scottish voters might agree. Remainers have tried to use the Scottish card to strengthen their hand in England by warning that Brexit would trigger a second independence referendum that a resurgent Scottish Nationalist Party (SNP) might win.

Nicola Sturgeon, the SNP leader and first minister of Scotland, has declared that Brexit would be a "material change" that could lead to unstoppable demands for another referendum. Yet there are reasons to doubt it would happen soon. The SNP remains dominant in Scotland, after sweeping 56 of the 59 Scottish seats at Westminster in the 2015 general election. But a month ago it lost its overall majority in the Scottish parliament, when the Conservatives leapt into second place. And even if she were able to call a second referendum, Sturgeon cannot risk it unless she is certain of winning.

Moreover, the uncertainties that defeated independence in 2014 remain. Oil prices are half as high as then, so an independent Scotland would face even bigger economic and fiscal difficulties. After Brexit, the E.U. might be more welcoming to a Scotland seeking membership, but it would still object to its keeping the pound instead of adopting the euro.

In contrast, Brexit would create immediate headaches for Northern Ireland, starting with the economy. Farming matters more in Northern Ireland than on the mainland, and it depends more on E.U. subsidies. Links to Ireland are crucial: it takes 34 percent of Northern Irish exports. Brussels has provided massive support to Northern Ireland since the Good Friday Agreement of 1998. Many in Belfast are skeptical of Leavers' promises to make up for any money lost by Brexit.

Ireland is the E.U. country most worried about Brexit. Irish ministers regularly state their opposition. Relations between the two nations are better than at any time in their history, and economic links have become closer. Travel and trade across the border are easier than ever. Britain is Ireland's biggest export market, and Britain exports more to Ireland than to China, India and Brazil combined.

Leavers say there is no reason why any of this should be affected by Brexit. Trade would continue. The common travel area between north and south began in 1922, not 1973.

Yet many in Belfast and Dublin find this attitude irresponsible. The common travel area worked only when both countries were either out of or in the European project, not when one was in and the other out. If a post-Brexit Britain restricted free movement or left the E.U.'s single market, there would be consequences for its only land border with another E.U. country, the 300-mile line dividing Northern Ireland from Ireland.

Nationalists in the north have already said that, post-Brexit, they would demand a referendum to redraw the border with the south. Memories of a hard border are unhappy.

The ultimate irony, says John Curtice of Strathclyde University, would be if, in an extremely tight vote, Britain ended up remaining in the E.U. only because nationalists in Northern Ireland, Scotland and Wales outvoted the English. That would surely rekindle English nationalism in virulent form, creating the biggest threat of all to the United Kingdom.

Copyright 2013 The Economist Newspaper Limited, London. All Rights Reserved. Reprinted with permission.