Corn poppy (Papaver rhoeas L.) is a problematic weed in cereal farms in Ireland, necessitating continuous herbicide use to protect crop yields. In 2022, poor control with acetolactate synthase (ALS) inhibitors and auxin-mimic herbicides was reported by growers/advisors in two resistance-suspect populations, namely PAPRH-R1 and PAPRH-R2. In this study, we quantified their resistance to both herbicide modes of action and investigated the underlying mechanisms. ALS inhibitor dose-response experiments revealed that both populations had ED50 and GR50 resistance indices (RI) above 120 to thifensulfuron + tribenuron, mesosulfuron + iodosulfuron + amidosulfuron and metsulfuron, and values of 4-13 to florasulam + pyroxsulam. Results of 2,4-D dose-response assays revealed higher resistance in PAPRH-R2 (ED50 and GR50 RI >9.1) than PAPRH-R1 (ED50 and GR50 RI >3.4). ALS gene sequencing detected Pro-197 mutations in all sequenced plants from both resistant populations. Pre-treatment with piperonyl butoxide (PBO, a cytochrome P450 inhibitor) reversed 2,4-D resistance with GR50, the more sensitive response parameter, decreasing from 680.8 to 84.9 g ha-1 in PAPRH-R1, and from 2508.9 to 456.8 g ha-1 in PAPRH-R2, both well below the recommended label rate of 1000 g ha-1. 14C-2,4-D assays ruled out reduced absorption or translocation as resistance mechanisms. Sensitivity screening further revealed incomplete control with other post-emergence foliar herbicides tested at half and full recommended label rates, except for fluroxypyr + halauxifen, which provided complete control of PAPRH-R1 at the full rate, and glyphosate, which provided complete control of PAPRH-R1 and PAPRH-R2 at both rates. This is the first report of multiple resistance in P. rhoeas in Ireland, involving ALS target-site mutations and likely P450-mediated 2,4-D metabolism. As cultural/non-chemical control of broad-leaved species is difficult, the continued availability of pre-emergence or autumn residual herbicides, together with effective auxin-mimics and stewardship practices, will remain essential for managing these species, including P. rhoeas, in cereal crops.