A senior IRA figure who was close to the Stormont talks between Sinn Fein and the British government is believed by gardai to have directed the Canary Wharf bombing.
The man, who is not an elected representative, originally played a key role in persuading IRA members to unite behind the ceasefire.
He has assumed responsibility for IRA operations in the past two years, although he is not thought to be a member of the army council" - the seven member group which would have endorsed the bombing, gardai believe.
He kept in close contact with the negotiations, which began at Stormont in December 1994 and reported his views to the IRA.
The man is known to be closely linked to another leading Sinn Fein figure who is believed to be on the "army council".
There are strong suspicions that the planning of Friday night's bomb attack, which killed two people and caused more than £50 million of damage in the London dockland area, originated in the Border area.
The IRA in Co Monaghan is believed to have been testing explosives in hilly country near Scotstown last month, when local gardai saw flashes lighting up the night sky and heard loud bangs.
Local officers reported that the IRA in their area and in Tyrone and Fermanagh were already on the verge of breaking the ceasefire.
Last week, gardai came across what appears to have been a recently used arms store outside Ardee, Co Louth.
Several men were reported to have been seen acting suspiciously and it is believed they were preparing to clear an arms or explosives dump.
The gardai also believe that the IRA has continued to smuggle weapons into the country and that it has been using members of other republican paramilitary organisations, such as the Irish National Liberation Army (INLA).
It is also understood that gardai in Dublin believe the call to the RTE newsroom alerting it to the end of the ceasefire may have been made from one of two public kiosks beside St Mary's Bridge in Drogheda.
The area around the kiosks was sealed off on Friday evening, and over the weekend they were removed to Dublin for forensic examination. It is also understood the gardai believe the same caller may have made other calls from those telephones at around the same time.
Also in Dundalk, on the by pass on Saturday evening, a man who appeared to be armed hijacked a black Porsche driven by a woman. He pushed her into the passenger seat, drove to a nearby estate, and ran off.
A senior garda in Dundalk said yesterday that his officers were advised to step up their "alertness" in respect of cross Border traffic.
Security was also stepped up at Dublin Airport in the immediate aftermath of Friday night's bombing in London. There are stricter parking controls in operation and cars approaching the main terminal are being searched.
Checks on cross channel ferry traffic are also being stepped up.
Garda Special Branch detectives who had been returned to ordinary crime and anti drug duties were being reassigned from the weekend to anti terrorist operations aimed at known IRA members.
"Operation Shannon", which has involved Garda Special Branch officers from Dublin mounting road checks between Loughrea and Galway to try to prevent roaming gangs from attacking elderly farmers, is being wound down and the officers reassigned to anti terrorist duties.
Gardai are worried that the IRA bomb in London will prompt loyalists to attack targets in the Republic. Orders to step up Border security significantly are expected to be issued this week.
Garda sources admitted at the weekend that they were caught completely unawares by the attack in London, even though there had been reports of unusual activity by suspected IRA members in the Border area in recent weeks.