The study presents the first technical and economic assessment of green hydrogen production large scale based on wind and solar resources in the Republic of Ireland. Three locations are chosen for the study, with different levels of solar radiation, and wind offshore potential nearby export harbours in Ireland. A complex python model HyWiS, Hydrogen Wind Solar, is built to simulate all energy subsystems, including renewable generators, large-scale proton exchange membrane electrolysers, desalination unit, hydrogen storage capacity of one day, and hydrogen pipelines. Temporal meteorological conditions are considered, a quantitative analysis is conducted to determine the levelised cost of electricity, and the levelised cost of hydrogen. Several scenarios at the least cost possible for green hydrogen production are assessed for 2030 and 2040, and the Cork location is determined as the least cost location, with a cost ranging from 3.53 €/kgH₂ to 2.75 €/kgH₂ respectively.