SECURITY sources in Northern Ireland and the Republic, were in agreement on one breaching last night about the security and bomb attack on the British army's headquarters in Lisburn, Co Antrim - it could only have been the work of the Provisional IRA.

The two republican splinter groups, the Irish National Liberation Army or the emerging Continuity Army Council group, have shown no sign of having the capability to breach the high security installation which is Thiepval Barracks.

The attack, if it is the IRA's work, is considered likely to trigger a return to violence by the loyalist paramilitaries.

British Army Headquarters Northern Ireland contains some of the most sensitive security installations in these islands. The former parkland around the red brick Edwardian buildings was gradually covered by featureless, some windowless, structures during the 1970s and 1980s.

Some of the roofs were covered in aerials which, it can be assumed, contained signals and other sensitive installations.

The Special Air Services and other specialist and controversial undercover units are believed to have had unit headquarters inside the base.

Thiepval also contains the office of the General Officer Commanding and the Commander Land Forces Northern Ireland, the first and second in command of the 15,000 strong garrison force.

The GOC and CLF traditionally reside in two villas behind the main information building in the complex centre. The bomb exploded about 100 yards from the houses.

The indigenous army unit, the Royal Irish Regiment, formerly the Ulster Defence Regiment, is also commanded from Thiepval.

The barracks, set amid the tree lined avenues and substantial middle class suburbs of Lisburn, has been at the top of the IRA target list for more than two decades.

Aside from some attacks in the early 1970s, before the barracks was properly fortified, its security had not been breached.

The bombing, therefore, fits in alongside the "spectacular" category of IRA bomb attacks on commercial and military targets in Britain, before and since the IRA ceasefire.

In a single stroke, it also makes up for the arrests and arms seizures suffered by the IRA since it returned to its campaign of violence, proving it can breach what was supposedly one of the most secure military locations in the West.

AT the most benign interpretation, the attack could be seen as an attempt by the IRA to show it is still capable of such a major attack before moving back towards a renewal of its ceasefire.

Senior security sources in the Republic last month insisted that the IRA was moving back towards a ceasefire. However, since then the Metropolitan Police Force recovered 10 tonnes of explosive and guns in IRA hides in London; the gardai uncovered a mortar factory, near the Border in Co Louth; and, now, the bombing has returned to Northern Ireland.

The bleaker interpretation is that the IRA has determined to restart the violence in Northern Ireland to precipitate a security and political crisis on the island.

Since the ending of the 18 month IRA ceasefire on February 9th, the loyalist paramilitary leaders have indicated that there was a "line in the sand" in respect of IRA attacks - which, when crossed, would restart loyalist violence.

WHILE the loyalists could not specify exactly what the "line in the sand" constituted, they generally suspected that it would probably be a "spectacular" inside Northern Ireland. The murder of an important security or political figure or a major IRA bomb attack on some important piece of commercial property was thought the most likely signal of a return to war inside Northern Ireland.

Loyalist leaders said earlier in the summer that a renewal of IRA violence and a consequent restarting of the loyalist campaign of violence was "not a question of if, but when". They had already decided that a return to conflict, no matter reluctantly, was inevitable.

More recently, loyalists have been predicting that the coming conflict could be as bad, or worse, than that which preceded the August, 1994, IRA ceasefire when the North experienced scenes of violence and levels of hatred and fear not experienced since the depths of the conflict in the mid-1970s.

This fact is well appreciated by the IRA leadership and yet it chose to mount an attack on a target that, would send shock waves throughout the security apparatus which supports Northern Ireland's position" within the United Kingdom.

The loyalists will be acutely aware that they must react or stand off.

Unfortunately, in loyalist eyes, the "peace process" which preceded the IRA ceasefire involved the political alignment of the Dublin Government, constitutional nationalist parties, including the SDLP, and the IRA's political wing, Sinn Fein.

The symbolic shaking of hands between Mr Gerry Adams, the then Taoiseach, Mr Albert Reynolds, and the SDLP leader, Mr John Hume, on the steps of Government Buildings, in Dublin immediately after the IRA ceasefire confirmed to loyalists that there was a "pan nationalist front" designed to undermine the position of Northern Ireland within Britain.

If they decide to restart their violence, an attack on a target in the Republic is a likely consequence. The Ulster Volunteer Force had just embarked on a campaign of attacks in the Republic when the IRA ceasefire was called in summer, 1994.

Since the IRA ceasefire ended, it is reliably understood, the loyalists have again been honing their bomb making skills.