Abstract Background Osteoporosis has a markedly increased prevalence in older adults with an associated risk of fragility fractures that can significantly impact on morbidity, quality of life and mortality. Despite this, osteoporosis is both underdiagnosed and undertreated and its true prevalence in older Irish adults is not known. In this study, we evaluate the prevalence of osteoporosis in Irish adults aged ≥60 years and also examine its determinants. Methods Participants were from a large cross-sectional study of older Irish adults aged ≥60 years. We excluded those with a known diagnosis of osteoporosis and on pharmacological treatment for same. We also excluded participants with major risk factors for osteoporosis (history of any previous fracture, aromatase inhibitors, androgen deprivation therapy and current or long-term glucocorticoid therapy >3 months). We used logistic regression to assess the relationship between osteoporosis (defined by WHO criteria—DXA T-score < −2.5 at lumbar spine, total hip, or neck of femur) and potential predictor variables. Results There were 1,089 participants. Mean age was 70.0 ± 6.2 and 59.8% were female. 15% had osteoporosis and 47% osteopaenia. The only positive independent predictors of osteoporosis were body mass index (p < 0.0001) and 25-hydroxyvitamin D (p = 0.007) while negative predictors were female sex (p < 0.0001), older age (p = 0.037) and glomerular filtration rate (p = 0.028). Conclusion Almost one in six older adults with no history of fracture or major risk factors for bone loss had osteoporosis. However, half had osteopaenia and overall, two thirds had low bone density. The major predictors of osteoporosis were consistent with other studies. As the mean age in our sample was 70, the prevalence may be much greater with advancing age. The findings support international guidelines which recommend routine DXA screening for women aged ≥65 years and men ≥70 years or at a younger age if risk factors for low bone mass.