They say the council’s decision on the flag, made possible by the fact that nationalists now hold 24 seats on the council, compared with 21 for the unionists, reflects the rapid growth of the Catholic population in the years since the Good Friday agreement, unsettling the long-held assumption among unionists that Protestants would constitute a permanent majority in the province.

The most recent census results, released last year, showed that 48 percent listed themselves as Protestant or brought up Protestant, down 5 percentage points from the 2001 census, while 45 percent of the population listed themselves as Catholic or brought up Catholic, a 1 percentage point rise. In Belfast, many say, Catholics are already a majority or nearly so and could form a majority across the province within a decade.

Since the Good Friday agreement specified that the province would remain part of Britain as long as a majority of the province’s people and of the population of the whole of Ireland wished it to be, the reasoning goes, Protestants who are resolved never to accept a united Ireland could be right in seeing the flag dispute as a harbinger of their worst fears.

Patricia MacBride, a Catholic whose father was killed by loyalist gunmen and who has been a leader in reconciliation efforts under the Good Friday agreement, said she had always feared that the peace process might founder when Protestants realized that the population numbers were moving against them. With the sense that power was shifting away from them, she said, the sense of betrayal among Protestants had intensified.

“There was always going to be something that triggered this upheaval,” she said. “Increasingly, they feel abandoned by the state whose agents they have been for so long.”

Strong backing for that view was evident on a recent morning in Belfast on Shankill Road, a depressingly run-down loyalist stronghold notorious as a center of sectarian ambushes, bombings and shootings during the Troubles. There the Union Jack was everywhere, atop buildings, in shop windows, in tattoos and in the hands of small children out shopping with their parents. In bars, cafes and shops, “the humiliation” of the flag issue was the center of conversation. In the winter chill, groups of men, mostly unemployed, reinforced one another’s indignation.