Gardai are still investigating claims by a former RUC member and convicted murderer, John Weir, that there was Northern security force involvement in the Dublin and Monaghan bombings in May 1974. Weir, who was convicted of murdering a Catholic chemist, Mr William Strathearn, in Co Antrim in 1976, has told gardai that the bomb was supplied by a member of the locally raised British regiment, the Ulster Defence Regiment (UDR).

Further Garda inquiries are to be made, including talks with RUC Special Branch officers who investigated the bombings.

The Irish Times has learned, however, that key forensic evidence relating to the timing mechanisms used in the bombs links them directly to dozens of other identical bombs used by loyalists in the 1970s.

No official forensic evidence has ever emerged about the timing and detonating mechanisms used in the three bombs that exploded in Dublin and a fourth in Monaghan town on May 17th, 1974, in which 33 died. However, both police and loyalist sources have confirmed the timing mechanisms in the Dublin and Monaghan bombs were identical to those used in many other bombings carried out by loyalists in Northern Ireland.

A climate of controversy has developed about the bombings, particularly in the last six years since a Yorkshire TV programme, The Hidden Hand, claimed that there was British security force collusion in the attacks.

Since then there have been repeated suggestions about "collusion" between British army figures such as the late Capt Robert Nairac and members of the Ulster Volunteer Force (the group which finally admitted the bombings in a statement two weeks after the Yorkshire TV programme). The claims are mainly based on private reports compiled by some figures who were involved in either the British or Irish security forces or police at the time, along with other circumstantial evidence.

The RUC Special Branch did draw up a list of potential suspects at the time, particularly for the Monaghan bombing, but this consisted largely of well-known loyalists in the Mid-Ulster UVF brigade. Several of these have since died, which allowed their names to be used freely in relation to the bombings.

However, sources close to the UVF leadership at the time have, for the first time, offered information which they say rebuts these claims.

In particular, they have described the timing mechanism which, they say, was used in each of the four bombs and which they also say was used in dozens of other loyalist bombings inside Northern Ireland and in the Republic. The timer was an alarm clock which, sources say, had the brand name "Jock Clock". The metal face of the clock was covered in electrician's masking tape.

These clocks provided two timer settings. If the hour hand was removed the detonator could be set up to 55 minutes in advance. If the minute hand was removed the explosion could be set for up to 11 hours in advance.

The detonating circuit consisted of a wire attached to whichever hand was left in place (in the case of the Dublin bombs it was almost certainly the minute hand, giving the bombers 50 minutes to cross the Border back into Northern Ireland) and to a pin pushed up through the tape-covered face of the clock. When the hand touched the pin the battery circuit was completed and the detonator ignited.

The source said the usual UVF bombs of this time contained identical detonating and timing mechanisms. Hundreds of these mechanisms were constructed and were very familiar to forensic and ordnance officers on both sides of the Border.

The Dublin and Monaghan bombs differed from the others used by the UVF inside Northern Ireland at the time in only one significant way, the sources say. The bombs used inside the North consisted mainly of fertiliser mix with a small "booster" charge of commercial explosive. This was because the commercial explosive was almost always in short supply.

The commercial explosive, in fact, may have been manufactured in the Republic, possibly at the production plant in Silvermines, north Tipperary. The loyalists acquired these explosives through contacts who stole the explosives from legitimate companies like quarry owners who bought quantities of the explosives before the Troubles worsened in the early 1970s.

The bombs in Dublin and Monaghan contained a much higher level of commercial explosive because this, a source said, was a "special" job. The bombs were designed to attack the southern State at a time when loyalists were mounting their successful rebellion against the power-sharing Assembly at Stormont. They were, in effect, a signal to Dublin that loyalists would not tolerate the cross-Border governmental element of the British-Irish agreed at the political talks in Sunningdale some months earlier.

In the loyalist "strike' of May 1974 the larger of the two loyalist paramilitary organisations, the Ulster Defence Association (UDA), played the role of imposing an effective blockade on economic life inside Northern Ireland, making internal government impossible. It was the UVF's role to attack the cross-Border element of the Sunningdale Agreement with the no-warning car bombings in Dublin and Monaghan.

The UVF, in its statement issued in July 1993, said the claims that the bombings were inspired by or carried out by or with the connivance of British security forces was "Walter Mitty" thinking. It added: "The type of explosives, timing and detonating methods all bore the hallmark of the UVF."

In a further recent comment on the claims about collusion, a senior figure in the organisation at the time made the point that no member of the UVF would trust any bombing proposal put forward by a member of the security forces; that British army or RUC accompaniment across the Border would be pointless and highly unlikely; and that, anyway, the UVF had already carried out several bomb attacks in the Republic since 1969 and had already proved it was capable of cross-Border bomb runs.

The loyalists do not dispute that members of the UVF and UDA were also in the local British army regiment, the UDR. Dual membership of the UDR and loyalist paramilitary organisations was commonplace, and dozens of UDR men were imprisoned for involvement in loyalist paramilitary crimes, including murder. Four of the loyalist gang which killed members of the Miami Showband in 1976 were in the UDR.

Other bombs detonated in the Republic by loyalists include:

October 19th, 1969 : UVF bomber Thomas McDowell killed while planting a bomb at the electricity station in Ballyshannon, Co Donegal.

December 1st, 1972 : Two busmen killed by a UVF bomb at Marlborough Street, Dublin.

December 28th, 1972 : Teenagers Geraldine O'Reilly and Patrick Stanley killed in bombs at Belturbet, Co Cavan.

March 7th, 1975 : fishing fleet in Greencastle, Co Donegal.

November 21st, 1975 : Aer Rianta employee killed by bomb at Dublin Airport.

December 20th, 1975 : two men killed by bomb in Dundalk public house.