Background: Irish school children have demonstrated poor proficiency in fundamental movement skills (FMS) and consistent with international literature, females and overweight children tend to be less skilled than males and non-overweight children. Interventions that are suitable for children of all abilities and which provide long-term improvements in FMS proficiency are warranted. Purpose: This study examined the immediate and long-term effects of an 8-week FMS intervention programme on 255 Year 3 and 4 Irish school children's (50% male, 7.4 ± 0.6 yr) FMS proficiency levels. It was hypothesised that using a mastery motivational climate to deliver the intervention sessions would provide immediate and long-term improvements for all children, including females and overweight children. Methods: Participants were conveniently recruited from 4 schools and randomly assigned to the intervention-control (Group I-C: 2 schools, n = 134, 48% male) or control-intervention (Group C-I: 2 schools, n = 121, 52% male) sequence. Group I-C completed the intervention (i.e. two 45-minute FMS classes per week in place of usual PE for 8 weeks) in phase 1, and after a 4-week washout, completed the control condition (i.e. routine PE lessons for 8 weeks) in phase 2, and vice-versa for Group C-I. FMS proficiency, assessed using the Test of Gross Motor Development-Third edition, and weight status based on body mass index (BMI) were recorded at 5 time points: pre and post phase 1, pre and post phase 2 and at 13-months post-intervention (i.e. follow-up). Results: Linear mixed models revealed significant group × time interaction effects for locomotor, ball skills and total FMS scores (all p < 0.001) following engagement in the FMS intervention. No significant changes were observed following engagement in the control condition (i.e. Group C-I: pre to post phase 1 and Group I-C: pre to post phase 2; all p > 0.05). Significant improvements for locomotor, ball skills and total FMS scores were reported for both groups at follow-up compared to baseline (all p < 0.001). No significant group × time × gender or group × time × weight status interaction effects were reported (all p > 0.05). The proportion of participants who improved from poor-mastery to mastery/near-mastery was significant for eight skills, immediately following the intervention and from baseline to follow-up. Conclusion: Significant improvements in FMS proficiency were observed following a short-duration intervention that was delivered by an instructor with specialist FMS knowledge and an ability to create a mastery-oriented climate during lessons. Although the long-term effectiveness remains unclear, it is likely that mastery-oriented PE lessons could facilitate greater improvements in FMS development for children of all abilities compared to traditional PE lessons. Future studies should explore if primary teachers feel they have sufficient confidence and pedagogical skills to support children's FMS development during PE.