This report presents an advanced Smart Irrigation System (S-iS) that leverages state-of-the-art artificial intelligence (AI) and the Internet of Things (IoT) to address global water scarcity in agriculture. The proposed system integrates next-generation multispectral and soil nutrient sensors with a hybrid communication architecture, utilizing NB-IoT/LTE-M for reliable wide-area connectivity and a LoRa-based mesh network for resilient, long-range field coverage. Powered by sustainable solar-energy harvesting, the system employs edge AI processing and fog computing to enable real-time, autonomous irrigation decisions at the node level, significantly reducing latency and bandwidth dependency. Advanced AI methodologies, including reinforcement learning and digital twin simulation, dynamically optimize water delivery based on real-time agro-environmental data, predictive weather modelling, and crop health analytics. Furthermore, the framework incorporates blockchain-verified data integrity and end-to-end encryption to ensure security and trust. Designed for scalability and farmer-centric usability through intuitive mobile and AR interfaces, this solution delivers a substantial reduction in water usage, enhanced crop resilience, and a measurable improvement in the water-energy-carbon nexus, presenting a transformative, sustainable model for precision agriculture in water-stressed regions worldwide.