Accelerating the transition to sustainable land systems requires integrated tools that can capture the complex interactions between soils, vegetation, livestock, and climate. In Ireland, grassland–livestock systems are a dominant land use and a major source of agricultural greenhouse gas (GHG) emissions. However, these systems also hold unique potential for delivering climate mitigation and ecosystem co-benefits. This study introduces a modular simulation framework designed to explore land use scenarios and carbon dynamics under changing climatic and management conditions. Built upon the HOLOS-IE platform, we develop and test interactive modules for grass growth and livestock productivity, calibrated for Irish agro-ecosystems. Unlike conventional approaches, our model links field-level management (e.g. grazing, fertilization) with soil carbon processes, allowing for a dynamic evaluation of nature-based solutions such as adaptive grazing, reduced-input strategies, and enhanced soil resilience. Spatially explicit weather, soil, and land use data are integrated to support scenario analysis and farm-level decision making. This approach contributes to global sustainability agendas by enabling climate-smart agriculture. It also aligns with principles of land governance through transparent digital tools that promote evidence-based policies and farmer engagement. The modeling system serves as a prototype for how agro-environmental models can be transformed into operational platforms for land monitoring, GHG reporting, and long-term planning under uncertainty. Our findings underscore the role of integrated modeling in delivering climate and biodiversity targets through sustainable land management.