Four out of every 10 adults left school before the age of 15, says the Conference of Religious of Ireland, in a call for measures to promote adult education.
Those who currently use adult education tend to be people who have already done well at school or at college, it says.
Publishing a document, Social Transformation and Lifelong Learning, Sister Teresa McCormack, director of CORI's Education Office, criticised the low level of investment in the sector.
The Department of Education and Science gives less than one-quarter of 1 per cent of expenditure to adult education, she said.
The document calls for guidance and counselling leading to individual plans for people who missed out on an education.
Participants could try a range of "sample or taster courses" before making their decision, it suggests.
This "lifelong learning entitlement scheme" would be aimed at three groups of people:
Young people who left school without completing the senior cycle.
People aged 25 to 40 who did not complete the junior cycle.
Any other adults who have literacy problems.
The document also calls for substantial extra funding for community education.
"Community education provides people in marginalised communities with the skills to analyse the problems which they are encountering in their daily lives and to engage in social action to tackle these problems," it says.
"It is, thus, as much about the development of communities as it is about the development of individuals." If the concept of lifelong learning is taken seriously by the education system, it will have profound implications, the document argues.
These include:
Less pressure on the curriculum if it is accepted that "all learning does not need to be packed into the years between five and 18 and based in schools".
An emphasis on students planning their own learning.
Alternatives to a subject-based curriculum, implemented through a timetable and delivered in 30- to 45-minute units of time.
CORI also wants a greater role for school-based assessment and a lesser role for written examinations.
"As a result of the emphasis on once-off written examinations the vast majority of students in post-primary schools spend most of their time studying traditional academic subjects, irrespective of their particular talents and interests," it says.
"Those who do not complete the full two- or three-year programme leave school with no credit for their learning and other achievements on that programme. These features of the system are damaging for all students but particularly so for those from disadvantaged backgrounds." Arguing for school-based assessments, it says: "In the case of many of the qualities which are considered most important, it is the student's own teacher who is best placed to make valid judgments".
The document was issued in advance of the National Forum on Adult Education in Dublin Castle later this month.