This Phase I trial will establish the recommended phase II dose of eribulin in combination with fixed doses of oral irinotecan in adolescents and young adults with relapsed or refractory solid tumors. Eribulin will be administered intravenously on days 1 and 8 of a 21-day cycle, while irinotecan will be administered orally on days 1-5. Patients will be assigned an eribulin dose level at the time of enrollment using a 3 + 3 Phase I design.