A new scheme is being opened today which will allow thousands of undocumented migrants and their families official permission to reside in Ireland.

Minister for Justice Helen McEntee has described it as a "once in a generation scheme" that would improve the lives of thousands of people who contributed to society but who still lived in the legal shadows.

According to the Department of Justice, there could be up to 17,000 undocumented people living in Ireland, including up to 3,000 children.

Speaking on RTÉ's Morning Ireland, Ms McEntee said that the scheme is not an open ended one and people will need to fit certain criteria in order to apply.

She said the aim is to process applications as quickly as possible and she hopes that some of the first people will get their decisions by the end of this year.

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Although applicants need to have been resident in Ireland for a four-year continuous period, the minister said that a potential for a window of 90 days has been built into the scheme, that would allow for someone to have left the State for valid reasons.

"Once you can show that you've had a four-year period upon documentation with potentially that small window, as I said, for humanitarian reasons if somebody has to travel home.

"We want to be as responsive as possible to people's situations; understanding that it's never straightforward," she said.

Meanwhile, those with children aged under 18 will only need to have resided in Ireland for three years.

Applicants are allowed to include a spouse or partner, and children aged between 18 and 23 in their applications. Successful applicants will be allowed to access the labour market and can begin a path to Irish citizenship.

Those with an existing deportation order or expired student permissions are also allowed to apply.

Applicants must be considered of good character and not pose a threat to the State, but having convictions for minor offences will not, by itself, result in disqualification.

Ms McEntee said there will also be an appeals process and people can be confident that if they are not successful that their application will be assessed independently under appeal.

She also said that a deportation order does not necessarily prevent a person from applying or being successful but that each individual and application will be vetted.

If there is a serious criminal conviction or otherwise outstanding, she said, then that is something that the Minister for Justice will have to take that into account.

Online applications for the scheme will be accepted for six months until 31 July when the scheme will close.

Migrants centre welcomes introduction of scheme

The regularisation scheme has been welcomed by organisations working with migrant communities.

A campaign called Justice for the Undocumented began from the Migrant Rights Centre of Ireland and has been running for 11 years.

"It started with a group of 4 or 5 undocumented people who were thinking about how to improve their situation. It has grown into a network of over 2,000 undocumented people who, over the years, have taken action and very bravely come forward to push for regularisation", says Neil Bruton, Campaigns Lead with the MRCI.

Mr Bruton says that the undocumented migrants here are just like the undocumented Irish in the USA.

"Undocumented essentially means they do not have status. Many would have before, but were unable to renew, but they decided to stay and try to make a better life here," he said.

CEO of the Immigrant Council Brian Killoran also welcomed the scheme, describing it as "rare" and said it would "change the lives of thousands of people" and "allow them to come out of the shadows".

However, he said there are some concerns that the criteria may be too restrictive and that an application fee may be prohibitive. The application fee for a single person is €550, while the fee for a family application is €700.

Mr Killoran said that some people who are undocumented might be close to destitution, and that there should be a fee waiver.

Mr Bruton meanwhile said they would have preferred the fees to be lower and said it would be difficult for some applicants to meet. He said the centre would try to support those people.

There are also concerns the scheme is not broad enough.

"They might have three-and-a-half years here, or they might have 4 years here with documents for some of that time", says Mr Killoran.

Mr Bruton agreed that the scheme "doesn't solve everything", but was a great first step.

"We look forward to working with the minister to building on it and hope it might be broadened in the future".

The Immigration Council says that people thinking of applying should seek advice through civil society organisations working with migrants like the Immigrant Council and the Migrant rights centre, as well as citizens information centres.