DUBLIN, Ireland ― The political crisis in Northern Ireland arising from a dalliance between a 59-year-old woman and a teenager has underlined the fragility of the power-sharing arrangements that were beginning to be taken for granted.

The affair, between Iris Robinson, wife of Northern Ireland’s First Minister Peter Robinson, and 19-year-old Kirk McCambley, has pushed Mr. Robinson to the point of resignation. If this were to happen, the whole edifice of shared government, painstakingly negotiated over years with the help of former U.S. Sen. George Mitchell and the British and Irish governments, could fall apart.

But the real issue here is not the sex scandal but the question of policing. Peter Robinson heads the pro-British and mainly Protestant Democratic Unionist Party (DUP) and governs Northern Ireland along with Deputy First Minister Martin McGuinness of Sinn Fein, which aspires to a united Ireland. Under the power-sharing deal, control of police and justice was to be devolved to the joint administration by the British Government. It was this promise that enabled Sinn Fein to persuade its followers to accept the partition of Ireland, for now, and enter a government in Belfast.

Unionists, however, are deeply unhappy at the idea of Sinn Fein — the political wing of the now-disarmed IRA — having any say in police matters. Peter Robinson has resisted pressure from London and Dublin to complete the process but there was an assumption that he would in time. The same cannot be said of any likely successor, since those breathing down his neck are more hard-line in their opposition to devolved policing.

Kirk McCambley, 21, leaves the Lock Keepers Inn, in south Belfast.

Paul Faith/PA Wire

The DUP is now in a terrible quandary. If it jettisons Peter Robinson and proposes a new First Minister unacceptable to Sinn Fein because of his or her opposition to devolved policing, then the power-sharing government falls and there will be a new election. However it would be awkward for the DUP, with its base of Protestant supporters who have conservative religious convictions, to canvass for votes in the wake of such a devastating sex scandal.

Much depends on whether Peter Robinson can survive allegations that he acted improperly in dealing with the financial aspects of the mess. Iris Robinson, herself an elected member of the Northern Ireland assembly, obtained 50,000 pounds ($81,000) from two property developers in 2008 to enable her young lover to launch a cafe. She failed to declare the gifts as required by law. The BBC, which broke the story last week, claimed that Peter Robinson also knew she had broken the rules and did not inform the authorities. Peter Robinson has asked a government lawyer to determine within a week if he is guilty of wrongdoing, and will resign if found at fault.

On Sunday Robinson asked a party colleague, Arlene Foster, to step in temporarily as First Minister, a procedural device while he tries to clear his name. Earlier, DUP colleagues gave Robinson a vote of confidence pending the outcome of the inquiry by the government lawyer into his actions.

The sex scandal has mesmerized Northern Ireland, particularly as Iris Robinson was a strident critic of other people’s moral standards. In the wake of the beating of a gay man in Belfast she notoriously declared that homosexuality was an “abomination.”

Her own affair began after her close friend, a Belfast butcher named Billy McCambley, just before he died in February 2008 asked her to look after his son, Kirk. Iris Robinson's friendship with Kirk developed into a sexual relationship and in June 2008 she arranged for her lover to set up a cafe beside a 200-year-old lock-keeper’s cottage on the river Lagan. Mrs. Robinson persuaded two property developers, Fred Fraser and Ken Campbell to give her 25,000 pounds ($40,500) each in checks made out to Kirk to start the business, and took 5,000 pounds ($8,100) in cash for herself.

When the young man, who is now 21, broke off the affair in December 2008, Iris Robinson demanded that he return the money. Negotiations were conducted through her political adviser Selwyn Black, who quit when he became fully aware what was going on and went to the BBC with the story. He provided a record of text messages and phone calls that indicate that Peter Robinson knew about the financial maneuvers. One text dated Jan. 14, 2009 from Iris Robinson to Black said that her husband “knows about the money … and he is attempting to keep his hands clean.”

Iris Robinson announced two weeks ago that she was quitting politics, and rumors of a scandal circulated until Jan. 7, when Peter Robinson gave an extraordinary interview to selected journalists in which he disclosed that his wife had had an affair and that he had forgiven her. He claimed he first knew about it on March 1, 2009, following which his wife attempted suicide.

The next day the BBC revealed the complete story. To make matters worse for Peter Robinson, it turned out that on the same day as Iris was being taken to hospital after attempting suicide, he took questions at the assembly in his capacity as First Minister and a video clip of him making a joke as he did so has portrayed him as cold and heartless. Irish Robinson has not been seen since the scandal broke but her husband claims she is getting treatment for psychological problems.