Senior anti-terrorist officers on both sides of the border fear a new surge of violence from dissident republicans.
Several key figures in dissident groups are due for release in the coming months.
They are expected to influence a reorganisation as they focus on the fallout from Brexit and the potential of it opening up new opportunities for them to recruit and raise funds in the event a hard border is introduced.
Most of the main groups are currently leaderless in the Republic and their hierarchies have collapsed because gardai have put terror bosses behind bars.
Internal factional fighting has also reduced their effectiveness.
Attempts were made by several groups, including the New IRA and the Real IRA, to find an overall leader who could unite them into one cohesive unit, but the talks foundered when agreement could not be reached.
The New IRA, which was formed in 2012 and is believed to be behind this weekend's car bomb in Derry, poses the biggest threat from dissident republicans since the Provisional IRA, according to an assessment by Assistant Commissioner Michael O'Sullivan, the head of the garda intelligence and security section.
That assessment is shared by the PSNI, which is also seriously concerned about the impact of a hard Brexit.
The New IRA's bomb-making skills are considered more advanced than those of other dissident groups.
The New IRA has also been finding new sources of explosives and weapons, and is continuing to recruit personnel not known by gardai or the PSNI to have been involved in dissident activities.
During 2016 and 2017, gardai seized 100kg of explosives, nine AK-47 rifles, a sub-machine gun and a sniper rifle from dissidents.
The New IRA has more than 50 activists, all of whom are listed as persons of interest to the garda security and intelligence section.
It is estimated to have about 200 supporters willing to provide logistical help.