Background Sleep, or a lack thereof, is strongly related to mood dysregulation. Although considerable research uses symptom scales to examine this relation, few studies use longitudinal, real‐time methods focused on pediatric irritability. This study leveraged an ecological momentary assessment (EMA) protocol, assessing bidirectional associations between momentary irritability symptoms and daily sleep duration in a transdiagnostic pediatric sample enriched for irritability. Methods A total of N = 125 youth ( M age = 12.58 years, SD = 2.56 years; 74% male; 68.8% White) completed digital, in vivo surveys three times a day for 7 days. For a subset of youth, their parents also completed the EMA protocol. Trait irritability was measured using youth‐, parent‐, and clinician‐report to test its potential moderating effect on the association between sleep duration and momentary irritability. Results Results from multilevel modeling dynamically linked sleep to irritability. Specifically, according to youth‐ and parent‐report, decreased sleep duration was associated with increased morning irritability ( b s ≤ −.09, p s < .049). A bidirectional association between parent‐reported nightly sleep duration and anger was found—increased evening anger related to decreased nightly sleep duration, and decreased sleep duration related to increased morning anger ( b s ≤ −.17, p s < .019). Trait irritability moderated this association, which was stronger for more irritable youth ( b = −.03, p < .027). Conclusions This study adds to the literature and suggests sleep‐irritability dynamics as a potential treatment target.