Purpose From September 2010 until November 2019, Ireland's infant vitamin D supplementation policy recommended administration of 5 mu g/day of vitamin D(3)from birth to 12 months to all infants, regardless of feeding method. This study aims to examine policy adherence. Methods In the prospective COMBINE birth cohort study (recruited 2015-2017), detailed longitudinal supplement data were examined in 364 infants across the first year of life, according to product type, dose, frequency, and duration. Vitamin D supplement use at 2, 6, and 12 months in COMBINE was compared with the BASELINE cohort (recruited 2008-2011,n = 1949). Results In COMBINE, 92% of infants initiated supplementation at birth. The median supplementation duration was 51 (40, 52) weeks, with a range of 3-52 weeks. While supplementing, most parents (92%) used an exclusive vitamin D supplement as recommended and 88% gave 5 mu g/day. Half (51%) gave vitamin D daily and a further 33% supplemented at least 3-6 times/week. Overall, 30% adhered fully to the policy, providing 5 mu g vitamin D(3)daily from birth to 12 months. A further 16% were broadly compliant, giving 5 mu g frequently for the full 12 months. Vitamin D supplement use at 2, 6, and 12 months in COMBINE was 93%, 89%, and 72%, considerably higher than our earlier BASELINE cohort at 49%, 64%, and 44% at the same time points (allP < 0.001). Conclusions We report a high level of vitamin D supplementation initiation at birth, with full to broad policy adherence among more than half of infants. There is scope to improve overall compliance by focusing on supplementation frequency.