The Irish Climate Action Plan (2021) aims to reduce greenhouse gas emissions from agriculture by 22–30% by 2030, including a 20% reduction in chemical nitrogen use. A field trial with a randomised plot design tested various bio-based fertilisers, from P-rich dairy sludges to N-rich poultry and pig manures, used alone or with synthetic fertilisers to meet crop N-P-K needs. Control plots received no fertiliser, others received synthetic-only inputs. Bio-based fertilisers increased soil organic matter (from 4% to over 5%) and improved P and K stocks. Yields were comparable to synthetic-only treatments. A 2019 maize trial estimated €130/ha savings when using bio-based fertilisers. These results show their potential to reduce input costs and emissions while sustaining yields. They also highlight the need for decision-support tools to guide farmers in choosing, combining, and applying fertilisers based on crop needs, nutrient balance, and environmental outcomes