Self-assessment practices have been advocated in recent Irish educational documents due to their potential to enhance school children's learning and self-regulatory skills. However, the literature has highlighted how some children struggle to make accurate self-assessments of their academic work, which diminishes such positive effects (Keane and Griffin 2015; Nicol 2009). Using Piaget's Theory of Cognitive Development (1970) as a theoretical framework, the present study sought to investigate whether children's academic self-assessments became more accurate in line with increased age and higher prior literacy attainment. Following training in the use of self-assessment writing rubrics, 85 school children from second class, fifth class and Transition Year wrote an English essay and later self-assessed their work using rubrics devised by Andrade, Du, and Wang (2008). Results indicated that overall, children's self-assessment scores held a weak relationship with their actual performance scores (r = 0.24). However, findings illustrated that children's self-assessments became significantly more accurate in line with increased developmental stages. Strong correlations also emerged between higher prior literacy attainment and children's accuracy in self-assessments, amongst second class (r = -0.45) and fifth class (r = -0.73) children only. The findings suggest that Irish school children, in particular, primary school children with low literacy attainment, display difficulty making accurate self-assessments of their academic work in literacy. Stemming from the research, implications for practice and future research directions are outlined.