The New IRA poses the biggest terrorist threat from dissident republicans in more than 20 years, Garda security boss Michael O'Sullivan has warned.
The assistant garda commissioner said that it was the most dangerous dissident group since the Provisional IRA.
Mr O'Sullivan told the Herald that the threat level from the dissidents in Northern Ireland was severe, which is the second highest threat level. In Britain, it is substantial, meaning there is a strong possibility of an attack.
This is its highest level since the Provisionals declared their ceasefire in 1994.
Mr O'Sullivan said that the garda assessment of the New IRA was shared by police and intelligence experts in Northern Ireland and Britain.
Intelligence services in both islands indicate that the group is monitoring movements of potential targets in the North and Britain.
The New IRA's bomb-making skills are considered more advanced than those of other dissident groups.
The group is also finding new sources of explosives and weapons and is continuing to recruit personnel not previously known by the Garda or the PSNI to have been involved in dissident activities.
An interception last month by the Garda Special Branch of a cache of TNT explosive in Dublin's north inner city is believed to have saved many lives.
Ballistic experts said the 4kg cache was enough to make 30 under-car bombs. Likely targets include police, military and prison officers in Northern Ireland.
The source of the TNT has not yet been established.
In the past two years, gardai have seized 100kg of explosive, nine AK-47 rifles, a sub- machinegun and a sniper rifle from dissidents.
As a result of a build-up in non-jury cases, a second special criminal court has been opened - something not seen during three decades of Provisional IRA terrorist activities.
Seventeen trials are listed for hearing involving 28 defendants, charged mainly with membership of an illegal organisation or possession of explosives.
One of the accused is charged with directing terrorism, the first time the charge has been brought since the conviction of Real IRA leader Michael McKevitt.
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, providing logistical help such as vehicles and safe houses.
The group was formed in 2012 and has strongholds in Armagh, Belfast, Donegal, Dublin, Louth and Tyrone.
It also has a stronghold in Cork, although it has been divided since the murder of Aidan O'Driscoll in December.
Its leader is a veteran republican from a Border county, while another senior figure is a former Real IRA boss in Derry. One of its key activists in Dublin has an address in Tallaght.
The New IRA comprises of former factions of the Real IRA, Republican Action Against Drugs - which was based around Clady, Derry and Strabane - and seasoned republicans who had previously appeared to support the peace process but became active again.
Other members include those who had stayed away from the Real IRA, the Continuity IRA and ONH (Oglaigh na hEireann) because of the infiltration of those groups by gardai and security agencies in the North.
Many members have acquired counter-surveillance skills by attending courses in countries such as the Czech Republic, Slovakia and possibly Poland, according to intelligence sources.
Most of them travel out under the guise of setting up supposedly legitimate security companies.
The New IRA has also been developing its "engineering" side and manufacturing improvised explosive devices and attempting to open up new routes to buy guns.
It was responsible in November 2012 for the murder of Northern Ireland prison officer David Black (52) on the M1 as he was driving to work at Maghaberry near Lisburn.
Some of its members were also involved in the 2011 murder of PSNI constable Ronan Kerr and the earlier murders of off-duty British soldiers Mark Quinsey and Patrick Azimkar as they took delivery of a pizza outside Massereene barracks in Antrim.
The New IRA has also been involved in several attempted killings and bombings in recent years. Its members are also behind a number of punishment beatings, which are at an all-time high in Belfast.
These are now referred to as community controlled violence (CCV) and have resulted in five deaths in Northern Ireland in the past year.
Gardai have kept a close watch on the movements of known New IRA activists, particularly during events such as the recent visit of Prince Charles.
A new Belfast-based dissident republican political party, known as Saoradh, which has the support of New IRA prisoners in jails on both sides of the Border, was launched last September .
Among those who attended its first ard fheis, held in Newry, was prominent dissident Colin Duffy.
A tribute was paid in a Saoradh newsletter to Micky Barr, who was shot dead in the Sunset House pub in Summerhill, Dublin, during the Kinahan-Hutch feud in April last year.
Saoradh also has a branch in Dublin.