This study investigates bullying incidence among mixed and single-sex post-primary schools, to explore if a correlation exists between bullying, self-esteem, mental health and affect. It is expected that individuals with previous or current harassment will show lower signs of self-esteem. This report predicted that there would be a statistically significant relationship between bullying, self-esteem, GHQ and mood. The General Health Questionnaire-12, The Revised Olweus Bully / Victim Questionnaire, Rosenberg Self-Esteem Scale and Positive and Negative Affect Schedule were issued to 113 post-primary school students. Comparisons were made across randomly selected participants from mixed and single-sex schools. Analysis was conducted through examining correlations and ANOVA procedures. Results found a moderately strong positive significant correlation between clinical disorders (GHQ) and negative mood. This report also found a positive correlation between bully/victim incidents and negative mood. There was no significance between bully/victims and self-esteem, positive mood and GHQ scores. Data collection for this study could help to design an anti-bullying programme for the schools involved.