DUBLIN, FEB. 20 -- More than two years after they signed the Anglo-Irish agreement, Dublin and London have hit a major roadblock in their efforts to cooperate on the future of troubled Northern Ireland.

The immediate focus of controversy is the accountability and reform of the provincial police force in the north, as well as the terms under which Britain detains terrorist suspects, and the procedures for requesting extradition of alleged Irish criminals.

But deliberations on these matters have been colored by centuries of colonial history and a profound mistrust that is never long absent from Anglo-Irish affairs. Regardless of London's intent, its actions in recent weeks have reinforced the longstanding belief here that no Irish Catholic, north or south, agreement or no agreement, can ever expect a fair deal from British justice.

At the same time, the current disputes have made clear that the accord was never designed as an equal partnership. The agreement, Britain insists, gave Dublin the right to make its views known on issues affecting the north, with no guarantee they would dictate British policy.

Although both sides say that their commitment to making the accord work remains intact, Irish Prime Minister Charles J. Haughey warned last week that "confidence has been seriously eroded," and declared that efforts between the two governments to resolve their differences had reached an impasse.

"Matters," he has said, "cannot be left where they stand."

In terms of their effect on Northern Ireland, Dublin believes that recent British law enforcement and judicial decisions have all but canceled the negative publicity awarded their joint enemy, the Irish Republican Army, following its bombing attack that killed 11 persons in the border town of Enniskillen last November.

The elimination of sympathy or more direct support for the IRA among Northern Ireland's minority Catholics was the fundamental goal of the agreement signed in November 1985. Equally important was the building of Catholics' trust in the largely Protestant Royal Ulster Constabulary (RUC), the provincial police force.

London promised that Dublin would have a consultative voice on behalf of the Catholics in matters concerning the two sectarian communities in the north, particularly those having to do with the administration of justice. In return, Dublin laid aside, while not relinquishing, its longstanding claim to Ulster's territory, and promised to step up cross-border police cooperation with the RUC.

Over the violent objections of Ulster's 2-to-1 Protestant majority, a permanent bilateral secretariat stationed Dublin officials in the northern capital of Belfast for the first time since the island was partitioned with Irish independence nearly 70 years ago.

Both London and Dublin believe some progress has been made under the agreement.

Discrimination in housing policy in the six northern counties has been alleviated and a longstanding prohibition against the display of Irish flags and emblems has been repealed. Two new Catholic judges have been appointed to Ulster benches, and security cooperation has brought the discovery of massive IRA arms caches on both sides of the border.

Protestant, or Unionist, resistance, long considered the principal impediment to the agreement, remains. But its efforts to paralyze the province's local government have largely collapsed as Unionist political ranks have been fractured by internal divisions.

All, in fact, seemed to be going smoothly until Jan. 25, when British Attorney General Patrick Mayhew announced the results of a four-year investigation into an alleged RUC "shoot-to-kill" policy that led to the deaths of six unarmed men at police hands in 1982.

The sensitive inquiry had been plagued by difficulties and reports of police obstruction for years, and was a regular item on the Anglo-Irish agenda.

Although no evidence of an organized police policy amounting to murder had been uncovered, Mayhew said, there was "evidence of the commission of offenses of perverting, or attempting or conspiring to pervert the course of justice" on the part of police officials.

But, he said, it had been decided that prosecution for these crimes would serve neither the "public interest" nor "national security." British officials strongly indicated that any such action might compromise vitally important informers and security operations in progress in the north.

In the Irish government view, the implication that the RUC was immune from punishment, even when guilt had been established, was an arrow straight into the heart of confidence-building hopes for the north, and efforts to eradicate long-held suspicions of the British in the south.

"It came without any warning, without any consultation," said a Dublin official. "It was an enormous blow to public confidence here. How could we justify continued cooperation with the RUC when it still contains officers against whom there was evidence like this?"

Dublin insists, and London denies, that the British had promised privately that there would be new prosecutions on the cover-up. The Irish have been refused permission to read the investigation report, which the Thatcher government insists is an "internal police document."

In the weeks since the Mayhew statement, three related issues have fueled Irish resentment and put strong pressure on Haughey to respond: A British appeals court denied a petition to quash 13-year-old convictions and life sentences against six Irishmen imprisoned for a bombing in the British city of Birmingham that they insist, and most Irishmen passionately believe, they did not commit.

The judges deemed as incredible new witness testimony that confessions had been beaten out of the men, indications that forensic evidence presented at their original 1975 trial had been less than conclusive, and the insistence of a member of the British Parliament, who had written a book about the case, that he had conducted interviews with the actual guilty parties.Britain confirmed news leaks of its plans to make permanent the temporary provisions of the 1974 Prevention of Terrorism Act (PTA), originally passed in response to public outrage against the Birmingham bombing, in which 21 people were killed.

The act, which allows police to detain terrorist suspects incommunicado for up to seven days, is seen in Ireland as an instrument of harassment against the 1 million Irish citizens residing in Britain, who have comprised the vast majority of PTA detainees over the past 13 years. Most are never charged.

"Whatever the purposes of this legislation," Haughey said last week, "its implementation has been discriminatory and insensitive and has caused widespread resentment among law-abiding Irish people who, rightly or wrongly, view it as racist in its operation." Britain has pointed out that while it plans to do away with the need to renew the act every five years, the PTA still will require annual authorization by Parliament.It was revealed in Dublin that the British so far have not complied with a new Irish law requiring them to submit evidence of a prima facie case and their intention to prosecute when making extradition requests to Ireland. Haughey had proposed the new legislation in December, as domestic compensation for his unpopular decision to allow formerly immune "political" defendants to be extradited to Britain.

Of six British extradition requests sent across the Irish Sea since the new law was passed, none of them has had the newly required documentation. British officials have called the requirements confusing and time-consuming, and have asked for talks on the issue.

Meanwhile, there are other indications that Britain, while not backing down on any of the main issues in the dispute with Dublin, is willing to make amends.

British Northern Ireland Secretary Tom King said Wednesday that a new investigation would determine whether internal disciplinary procedures should be invoked against any of a reported dozen or more RUC officers allegedly involved in a cover-up, including the force's three top officials.

Making public what London and Dublin had previously decided to keep secret, King also announced that the RUC Special Branch involved in the 1982 killings had been reorganized and placed under strict control.