Undocumented migrants are more likely to face material deprivation and are more vulnerable to exploitation and crime than legal residents, according to research.
The joint research was carried out by the European Migration Network (EMN) and the Economic Social Research Institute (ESRI).
The study found that most undocumented migrants in Ireland entered the country legally, and later became undocumented. The majority reside unknown to the authorities.
The EMN and ESRI also found that no official data exists on the size of this group and estimates are difficult.
In 2020 the NGO, Migrant Rights Centre Ireland, estimated that there were 17,000 - 20,000 undocumented persons in the State, including 2,000 - 3,000 children.
The organisations looked at policy on undocumented migrants in Ireland and found that regularisation is one policy response to the issue of irregular migration, either on a case-by-case basis, or in the form of collective regularisation programmes.
Ireland's first scheme for undocumented former students was introduced in 2018 and just over 2,250 residence permissions were granted.
The 2022 Regularisation of Long-Term Undocumented Migrants Scheme is open until 31 July.
Adults who have been undocumented in the State continuously for the previous four years are invited to apply.
It is reduced to three years for families with children.
A scheme covering International Protection applicants who have been in the process for at least two years was also launched.
While the research found that undocumented migrants may not access social housing; they can access emergency healthcare and basic medical care with payment.
However, NGOs indicated that access to more specialised healthcare has been difficult for this group. Access to mandatory education is universal.
Previous research shows that fear of detection can lead to underuse of public services.
Co-author of the report Michal Polakowski described the regularisation of the Long-Term Undocumented Migrants Scheme as an important opportunity for undocumented migrants to join Irish society, to live and work here legally, and to access services.
However, he said undocumented stays are likely to continue in the future.
"Going forward, it is important that all efforts are made to avoid people falling into this vulnerable situation," Mr Polakowski said.
"Also, in cases where a migrant's residence becomes undocumented, care should be taken to ensure they are not afraid to access basic services."