Inflexibility of conventional power plants is one issue, but for Ireland things has developed to a point I suspect it is no longer the main operational constraint on the grid.

Ireland is an island grid and needs to keep system inertia on its own. This service is traditionally provided by conventional power plants in GW scale grids, but soon when synchronous condenser and inverter-based solutions become norm, there is no reason why 100% instantaneous wind + solar is not possible as shown already in various microgrids.