This thesis evaluates the operational resilience of the All-Island transmission grid (Ireland and Northern Ireland) under future scenarios of high and low renewable energy penetration. Using DIgSILENT PowerFactory, a validated static power flow model for 2035 was developed based on the 2024 grid, incorporating realistic projections for renewable deployment, interconnector expansion, and demand growth in line with national and European targets. Simulations were conducted for a range of seasonal demand scenarios, distinguishing between high and low wind capacity factors. The results show that high renewable scenarios induce significant grid stress, particularly under low load and high export conditions, with congestion concentrated along major eastward corridors.