Ireland will start using the main EU police database system on 1 January, the same day that the UK stops using it.

The Schengen Information System (SIS II) is the largest and most widely used IT system for public security in Europe, with 30 countries participating.

Ireland and the UK had initially stayed out of the system, which was started in the 1990's.

The UK has been a member of this system since 2015, but Irish entry into it was delayed by spending cuts in the wake of the financial crisis. 

Since then, the UK has become the third most active user of the system, which provides police with real time access to data alerts on criminal suspects, missing persons, stolen vehicles and property.

British police forces and security agencies say losing access to the system as a result of Brexit will be a major setback to fighting criminals, terrorists, paedophiles and people traffickers. 

Records for anyone sought for arrest and surrender for extradition across Europe will be automatically checked each time a passport is scanned at a point of entry to Ireland, or a name searched on the Garda PULSE system.

Passport scans will be checked for anyone sought for arrest

All lost or stolen objects/vehicle records created by An Garda Síochána will also be immediately available to 30 EU and associated Schengen countries. 

At the end of last year there were some 40,000 wanted persons data in the system, and 120,000 missing persons alerts.

Last month, EU ministers cleared the way for Ireland to start using SIS II on 1 January, with full operational capacity by March.

It will mean passport checks at the Irish frontier will provide law enforcement agencies with real time information on people accused or convicted of crimes in other EU countries, along with Norway, Iceland Switzerland and Lichtenstein which all apply European freedom of movement rules. 

A report on Ireland's membership of the Schengen II system, on the very day that Britain leaves it by the UK parliaments House of commons European Scrutiny committee, says the timing of the decision is significant. 

It stated: "It sets in motion a process for Ireland to participate in SIS II which mirrors a reverse process in the UK. The Government recognises that the UK's exit from SIS II at the end of 2020 will result in a mutual loss of capability both for the UK and the EU and, for that reason, is seeking to develop other channels for sharing information."

Police leaders in the UK have indicated that losing access to SIS II will have "a major operational impact" because the information it provides has become an integral part of policing systems across the EU. 

The Committee's report says it will take time to find alternative means of co-operation that are equally effective. 

Meanwhile it says "the impact of new customs arrangements and regulatory requirements under the Protocol on Ireland/Northern Ireland on cross-border criminality on the island of Ireland (and the wider ripple effects for the UK as a whole) remains uncertain". 

Former Secretary of State for Northern Ireland James Brokenshire

Referring to a memorandum on the Irish admission to the SIS II system by the UK security Minister (and former Secretary of State for Northern Ireland) James Brokenshire, the Committee report said it was "disappointing that the Minister does not consider whether Ireland's participation in SIS II may have implications for the UK, particularly in underpinning cooperation between the Police Service of Northern Ireland and the Irish Garda in tackling cross-border crime after transition". 

It added that "while there will be legal constraints on the information that Ireland will be able to share with the UK, there may nonetheless be some operational benefits for the UK when it loses access to SIS II, just as there may have been for Ireland when the UK had access to SIS II", suggesting that informal police contacts on both sides of the land border will enable information sharing outside the automated system (even though the report notes that the Schengen regulation explicitly bans the supply of SIS II data to third countries, which the UK will be after 1 January). 

Ireland remains outside the Schengen common travel area, but takes part in some policing co-operation arrangements that are part of the Schengen Agreement.