Gardai are concerned that a unit of the dissident republican group, the "Real IRA", has established itself in England and was responsible for yesterday's bombing at Hammersmith Bridge.
Both the Garda and RUC have become anxious in recent weeks that the dissidents have been gearing up for attacks.
The main concern was that the group was attempting to kill members of the security forces in Northern Ireland, mostly probably in a mortar or bomb attack on a British army or RUC base.
Gardai have also learned that the "Real IRA", which was responsible for the Omagh bombing in August 1998 in which 29 people were killed, has recruited new young members with no previous history of involvement in terrorist activity.
In the past week gardai arrested 13 people in raids throughout the State. Among those held for questioning were young people with no record of involvement in republicanism. One of those arrested was only 17.
It is also understood that the group has recruited the services of former senior Provisional IRA figures who are opposed to the direction taken by the Provisional republican leadership on participating in the Stormont Assembly and agreeing to the inspection of arms dumps in the Republic belonging to the Provisional IRA.
There have been concerns for some time that the dissidents would try to relaunch a bombing campaign in Britain.
They previously tried to do so in the early summer of 1998 and recruited young people with no previous history of involvement in republican activity.
The unit was sent to London with Semtex explosives and incendiary devices which were intended to be used in city centre stores. But they were caught after the organisation was infiltrated by gardai.
The group would have been capable of manufacturing small bombs like the one which exploded under Hammersmith Bridge.
However, in recent months several attacks in Northern Ireland have been attributed to the "Real IRA". It appears that the Garda is having less success in infiltrating that organisation.
The timing of the Hammersmith Bridge bombing on a key day in the Northern political process immediately directed suspicion to the dissidents.
The last attempted bombing by the Provisional IRA in England, before the August 1997 ceasefire was called, was at the same bridge.
On that occasion the Provisional IRA planted a 30lb Semtex bomb under the bridge. Had it exploded, it could have brought the entire structure down and caused traffic delays in central London for many months.
There are suspicions that the dissidents may have been sending out a message that they intend continuing the IRA bombing campaign from the point at which it stopped.