She described having a strict daily routine through her teenage years that consisted of going to piano lessons, going to church and doing homework. O'Riordan later admitted that she had neglected her school lessons in favour of writing music and songs, although at school she became head girl. Former principal Anne Mordan said in Nova about O'Riordan that she was a "delightful, unsophisticated, sensitive student, who enjoyed her time with us"; she described her as "a bright, kind, good-humoured girl, who loved her family, her friends, and had an easy relationship with all her teachers, both lay and FCJ sisters." During her six years at Laurel Hill Coláiste, O'Riordan won the Slógadh song contest almost every year, at several local events, and culminating in national singing competitions. In total she won 20 Slógadh medals.
