Reanalysis is a technique used to reconstruct past climate using a state of the art Numerical Weather Prediction system. Met Éireann has recently completed a high-resolution reanalysis of Ireland's climate, called MÉRA. This reanalysis assimilated in-situ observations over the period 1981-2015 using three-dimensional variational data assimilation (3D-Var). An overview of 3D-Var and validation results from MÉRA are presented.