I get this pattern like artifacts when using irradiance baking and if I smooth it out, the whole thing gets blocky. Is there a setting to get more accurate result with something like basic linear interpolation?
The irradiance volume is made of several samples (resolution).
That is blending of all this samples that is giving final result.
Basically, every setting has an impact on blending of samples.
The resolution is influencing amount of samples but also their distribution in space.
Intensity and Clipping settings will set how noticeable a sample will be.
Increasing amount of pixels of Diffuse Occlusion maps may result in visible artefacts in blending of few samples, if there is no blur.
On contrary, blur will have a smaller effect on Diffuse Occlusion maps of low resolution. But if there are enough samples, Irradiance Smoothing of cache should be effective.
Amount of bounces, importance of bias and bleeding may lower or increase perception of artefacts according to scene.
Obtaining an irradiance map, that is not to long to render and without artefacts is a question of balance between settings.
In cache of just two closed objects in an interior, you could obtain a satisfying result, without an irradiance probe, just by tweaking Ambient Occlusion.