The present study examines the role of irritability in form and function subtypes of aggression over 1 year in early childhood (N = 300, Mage = 44.70 months, SD = 4.38 months). This study prospectively tests hostile attribution biases (HAB) as a mediator in irritability’s relations with aggression, with hypothesized form-specific relations between HAB and aggression. Moderation by gender and a reversed alternative model (aggression to irritability, mediated by HAB), were also tested. Path analyses showed irritability predicted increases in all subtypes of aggression (βs = 0.24–0.34), but with moderation by gender, such that irritability significantly predicted increases in reactive relational aggression for girls only (β = 0.43). Reactive physical aggression also significantly predicted increases in irritability (β = 0.15). HAB was not associated significantly with any forms or functions of aggression, although gender differences emerged between HAB for instrumental provocations and reactive physical aggression. No significant indirect effects were found. Results highlight the importance of considering both forms and functions of aggression when investigating irritability, and point to potential gender differences in the role of irritability in relational aggression in early childhood.