Official figures show that two thirds of the 1m foreign nationals who registered to work in the Republic of Ireland between 2004 and 2009 were no longer in the official workforce in 2009.
They also indicate that as many as two thirds of new arrivals in 2009 may have moved elsewhere by the end of that year.
The Central Statistics Office has been tracking the numbers of foreign nationals who were assigned personal public service numbers each year as well the proportion of those of working age who remained in the official workforce.
This means any PAYE employment or any engagement with the social welfare system.
It has found that of the approximately 1m who registered to work during the period 2002-2009, just over one third were in the official workforce during 2009.
The CSO says that a small proportion of those no longer in official employment could have set up their own businesses or entered full-time education without taking up any job in the PAYE sector.
Even allowing for a movement by some into the black economy, it appears that some as many as 600,000 of foreign nationals who arrived during the five years up to 2009 left the jurisdiction by the end of that year.
By then, PPSN allocations to citizens of the 10 countries which joined the EU in 2004, had slumped to one-fifth of their 2006 peak.
