IT DID NOT take long for the Commissioner of the Irish police and the Chief Constable of the Royal Ulster Constabulary, meeting in Belfast last Monday, to decide both what their strategy against the "Real IRA" should be and the measures to achieve their goal. Within a couple of hours the two men were on their way to Stormont with what one security official describes as a "shopping list" for their political masters.

The two security supremos want to decapitate the Real IRA. To do this they need to tackle a problem as old as the Troubles themselves: that what they know, in the form of intelligence, is quite different from what they can use as evidence in court.

In formulating the strategy, they took advice from their intelligence organisations. Their assessment, a security source says, was that the Real IRA is "a mixture of very experienced godfathers with all sorts of expertise in bomb-making, [and] foot soldiers who have no experience whatsoever and are previously unknown". There is no "middle management".

Both the RUC and the Garda have accumulated plenty of information about those "godfathers", whose ranks include the former quartermaster of the Provisional IRA and head of its England department. Their belief in the amateurishness of the movement's functionaries is derived from several operations including the attempted firebombings in London two months ago and the Omagh bombing itself.

Two steps announced last Wednesday by the Irish Prime Minister are vital to neutralising the Real IRA leadership: it will be easier to convict somebody of membership of a proscribed organisation on the word of a senior police officer, and Ireland will introduce a new law, modelled on one the UK put in place three years ago, making it an offence to "direct terrorism". The first involves toughening up clauses of the Offences Against the State Act 1972, and both are designed to bridge the gap between intelligence and evidence.

Of the other steps taken by the Irish government, the decision to allow terrorist suspects to be held for up to seven days appears to be aimed at the Real IRA's foot soldiers. The UK has been able to do this since the Prevention of Terrorism Act 1974. When that was introduced, its most dramatic effect in Northern Ireland was to allow the breaking under interrogation of the weaker-minded suspects, people who might have been able to remain silent for the customary 48 hours.

If these measures and the creation of an offence of "directing terrorism" seem like the Republic catching up with British counter-terrorism practice, it is also true that Ronnie Flanagan, the RUC Chief Constable, would like to copy the Irish measures to imprison suspects on the word of a senior police officer. Tony Blair's suggestion on Thursday that there would soon be changes to UK law suggest Mr Flanagan has won this argument.

ALL OF THESE new measures will put the onus on the intelligence operators to deliver the information needed to put the right people behind bars. There would be great embarrassment if a wave of arrests of Real IRA "godfathers" were followed by reprisal attacks from some still at large. Everybody involved in counter-terrorism knows there can be no repetition of Britain's disastrous 1971 internment which rounded up more than 340 people but left most of the key Provisional IRA leaders untouched. The experts, particularly in the RUC Special Branch, have come a long way since then though.

The Dublin authorities also say they expect to expand their surveillance operations in order to get better information. Here once more there is a sense of a catch-up: the Garda's Surveillance Unit is only a few dozen strong. The RUC Special Branch has its E4A surveillance section, about 50, the Army's elite unit codenamed 14 Intelligence Company, with around 60 soldiers, and other less highly trained Army reconnaissance platoons able to bring another 150 or so operators to rural areas. But even though the security forces in Ulster might have some 260 surveillance specialists, they know that watching people continuously is an extremely labour-intensive activity.

In 1993, for example, MI5 began Operation Mileage, which eventually resulted in the conviction of three IRA men living in Scotland and north- east England. It involved blanket surveillance of the suspects for months; when two of the bombers went on trial, there were 84 MI5 witnesses and dozens from different police forces.

Surveillance of a single suspect in Ulster, too, can tie down one of the three detachments of 14 Intelligence Company since it will have only around a dozen soldiers available at any given time and they need to be spread between static observation posts, vehicle and on foot. In practice the Army and RUC's covert operations have shown that leading terrorists cannot be followed around indefinitely and that intelligence, usually from informers, is vital to trigger worthwhile surveillance.

RUC and MI5 operations such as Mileage have also shown that careful terrorists can elude such high levels of activity. On several occasions in 1992-94, bombs or bomb-making materials were delivered to IRA men being watched by MI5 in Britain without the security agencies being able to spot the couriers. In the case of the planned Gibraltar bomb in 1988, too, British and Spanish authorities never discovered where and when the device was brought into Spain. With the Real IRA there is nervousness in security circles on both sides of the border about just what the organisation's leader, the former Provisional IRA quartermaster, may have spirited away before the split.

The Omagh bomb is thought to have contained 500lb of home-made explosive. The earlier Real IRA bombs in Banbridge and Newtownhamilton used Semtex to boost these readily available ingredients to the right destructive power. How much of the Provisionals' Semtex and other equipment are under Real IRA control? Intelligence chiefs don't really know.

In the key areas of estimating available Real IRA weaponry, pinpointing its leaders and getting the right tip-offs needed to conduct effective surveillance, the co-operation of Sinn Fein could make the difference between success and failure. Throughout the past week, leading members of Sinn Fein have been facing tough questions ranging from whether they will provide information to the RUC to whether the Provisionals are ready to assassinate the dissidents.

SINN FEIN faces enormous risks if it co-operates in the elimination of the Real IRA. Any assassination of the latter's leaders could see the party ejected from the peace process. As for turning publicly on the dissidents, not only is Sinn Fein committed to disbanding the RUC but encouraging witnesses to give evidence in court would be seen as a betrayal of decades of IRA resistance. Certainly the RUC's Chief Constable is not expecting any help of this kind: another of the measures he is believed to have suggested to the Government is a law that could compel witnesses to give evidence. All of which leaves intelligence, the kind of tip-offs Sinn Fein can hand over, or turn a blind eye to other nationalists handing over, without it becoming public that the party has done so.

Any help from Sinn Fein will have to be very discreet indeed since the party runs the risk of being seen to do "the dirty work of the British state", as one former Provisional put it last week. Security chiefs clearly believe the gamble of trying to imprison the Real IRA's leadership is worth it and have sought the measures to enable them to do it. Make no mistake, it is a gamble though, both because patches in their intelligence may leave certain terrorists at large and because a security drive seen in republican communities as a systematic violation of civil liberties might serve simply to drive more recruits into the dissident groups.

Mark Urban is a correspondent for the BBC's 'Newsnight'.