Thirty jihadi fighters have been established by Garda intelligence to be using the Republic as a base while travelling regularly to conflict zones in Syria and Iraq.
The group are being kept under close surveillance by gardai and are also under watch abroad by international police agencies. But senior officers admit they cannot assess the extent of their involvement in the fighting when they enter the strife-stricken countries.
Gardai have now stepped up surveillance efforts as part of an EU-wide move to track their movements and identify associates as fears grow over radicalisation and security risks they pose when they return from the conflicts.
Officers are also forging closer links with the Muslim community in the Republic to unearth any signs that the fighters are trying to radicalise a new generation.
Three of those known to have travelled to conflict zones have been killed, including a 16-year-old boy.
But so far there is no indication that recruitment in the Republic is on the scale seen elsewhere in the EU. And the overall assessment is that the threat to Ireland from international terrorism remains low.
And while the 30 known to be based in the South are top of the Garda watch list, they comprise a small percentage of the 50,000-strong Muslim community there.
Most of those under surveillance live in the greater Dublin area and two kept under constant watch are regarded by international agencies as key logistic figures.
But the biggest fear of security chiefs more recently has been the potential emergence of previously unknown youths, radicalised by hard-liners and turned into jihadi fighters.