More than 100,000 children are missing over 20 days of school each year, a new report from the National Educational Welfare Board shows.
The NEWB's Annual School Attendance Report also found attendance was still a problem, particularly among older children and those from more disadvantaged and urban areas.
More than 55,000 students are missing school each day, broken down into 27,000 primary and 28,000 post-primary students. This represents a loss of 12 school days per student per year for primary schools, and 13 school days at post-primary level.
A little over one in 10 primary school students and one in six post-primary students are missing 20 days or more during the school year.
The report highlighted links between poor attendance and what it described as "other indicators of educational disadvantage". It said primary schools that experienced high levels of absenteeism were likely to have a high proportion of students living in local authority accommodation, in lone-parent families, or in families where the main earner is unemployed.
Meanwhile, non-attendance for post-primary students was linked to high ratings for socio economic disadvantage. There were high rates of dropout in both junior and senior cycle, and poorer performance on the Junior Certificate Examination.
Chief executive of NEWB, Eddie Ward, said absenteeism was "unacceptably high".
"Combating the issues underlying poor attendance requires concerted action and further integration of existing school support services and other stakeholders involved with children, young people and their families," he said.
"The report tells us that absences of 20 days or more are a central feature of social exclusion which if measured over time will give us a true indication of the progress being made in addressing disadvantage in education."
The president of the TUI, Don Ryan, said the number of children missing more than 20 days a year was worrying.
"The reiteration that the non-attendance is linked to socio economic disadvantage and poor performance in State examinations should draw the government back to reality and show how truly shameful some of the recent budget cutbacks are," he said.
"Data of this significance is a reminder that despite the economic downturn investment in education is paramount to counteracting deepening social inequity and inequalities.
"The TUI anticipates that these problems will intensify, resulting in irreparable harm to the prospects of a whole generation of school-goers, unless the worst of the budget cutbacks to education are reversed," he added.
The NEWB report also found that reported expulsions are still uncommon, with only 134 recorded in 2005/6. Sixteen of those were in primary school with 118 at post-primary level. There was a higher level of suspensions in post-primary schools, with more than 16,000 suspended in 2005/6, compared to over 1,000 at primary level.
The report covered attendance in primary and post-primary schools from 2003/4 to 2005/06.