A clampdown on illegal immigration, especially organised scams such as sham marriages, has resulted in a very significant increase in the number of residency permits being refused or revoked.
The number of deportation orders being signed has also increased.
The Department of Justice said an “organised and concerted attempt to abuse the system” had been targeted.
Thousands of people who were trying to come to the Republic have simply withdrawn their applications, as word has spread about the enforcement in Ireland.
Garda and justice sources believe the numbers involved prove the system was being abused by a large number.
However, the same sources said the vast majority of foreign nationals living in Ireland had gone through the correct channels, were fully entitled to be in Ireland and were compliant with the rules.
The number of people, most of them men from Asian countries, being investigated for suspected participation in sham marriages in Ireland has doubled in two years.
Some 625 men who secured immigration status to live and work in Ireland by marrying a woman from an EU member state have been investigated.
And that figure is expected to continue increasing as about 2,000 sham marriages are believed to have taken place in total. The marriages were being organised by criminal syndicates based in Ireland and abroad.
Men from Pakistan, India and Bangladesh and other Asian countries married women from Portugal, Latvia, Estonia, Slovakia and other European countries. On average, a fee of €20,000 was paid to the facilitators with the woman, many of whom were impoverished, being paid €2,000 or less.
As well as sham marriages, gardaí have also uncovered immigration-based scams aimed at trafficking foreign nationals for labour purposes into the fishing industry, to work in car washes and also in nail bars.
Others have been brought into the country for the purposes of sexual exploitation.
The EU Treaty Rights Investigation Unit of the Irish Naturalisation and Immigration Service (INIS) within the Department of Justice has initiated 2,525 investigations since 2015.
In 1,533 cases, permission that had been granted to foreign nationals to reside in Ireland was revoked, because new information uncovered on investigation revealed their applications were based on fraud.
And 327 applications were refused on the same grounds, meaning the fraud was identified before immigration status was granted.
However, sources say the extent to which the immigration system was being abused is clearest when EU Treaty Rights visa application trends are examined. When foreign nationals newly secure an immigration status that allows them to live in Ireland, including marrying an EU national, their family members in other countries can apply for a visa to come and join them in Ireland.
In 2015, as Operation Vantage and other investigations into immigration abuses intensified, applications for the visas had been significantly increasing. In 2015, for example, there were 9,968 applications.
But as the investigations began to have an impact and word of them spread, the applications began to fall: to 7,127 in 2016 and down to 2,800 in both 2017 and last year. “There were significant concerns that these represented an organised and concerted attempt to abuse the system,” the Department of Justice said in reply to queries.