The row over Aer Lingus's decision to transfer its Shannon-Heathrow routes to its new UK hub at Belfast International and to employ new pilots at UK rates has escalated into a full-scale crisis. The airline will lose millions of pounds in revenue, if the pilots' strike goes ahead, and the Dublin government may be forced to intervene if opposition from west of Ireland interests hardens.
When Ian Paisley and Martin McGuinness hosted the announcement that Belfast had won the contest for the airline's new base in the UK, it was greeted as a by-product of the peace process, confirming that Northern Ireland was open for business with the Republic or anywhere else. At least 100 jobs would be created directly, and there would be many more from increased tourism.
But when it became obvious that Belfast's gain was Shannon's loss, and that the renewal of Aldergrove's link with Heathrow would mean an end to the Shannon-Heathrow route, the picture changed dramatically. The west was awake to the adverse effects on a fragile economy and not only TDs and councillors, but priests and archbishops got involved in the protest.
The revelation that pilots and staff working from Belfast would be paid on a different basis to their Dublin counterparts generated more opposition, bringing about a threatened strike and disrupting the holiday plans of thousands. The pilots and the politicians could unite, even though they were taking on a privatised airline.
The reason for the privatisation, of course, was that Aer Lingus was losing huge amounts of public money and needed the kind of shake-up that the establishment of the UK hub envisions. As an independent company, working for its shareholders - including Ryanair's Michael O'Leary, with a 25% stake - it has to be free to put its money where it will get the best returns.
The opportunist Mr O'Leary, who failed to buy Aer Lingus, has added to the confusion by calling for an extraordinary general meeting, opposing the ending of Shannon's Heathrow service. His recommendations would allow the Belfast-Heathrow flights to begin next January, but would mean unpopular reductions in the service from Dublin and Cork.
The Taoiseach, Bertie Ahern, who is on holiday, has been keeping his head down, but he may be forced to make a statement if the protest movement that is threatening his party and government continues to grow. Having privatised Aer Lingus, he must allow it to take commercial decisions, or Ireland's reputation as a free market economy would be at risk.
With hindsight, the politicians at Stormont might have been wiser to keep politics well away from what was a controversial, though profitable, transaction. This time, Northern Ireland is the beneficiary, but north-south trade, in the post-Troubles era, is bound to work both ways.