Is there a way to have a tiling texture for say a brick wall, and place a non tiling png with alpha channel as splatmap or decal?

In Iray images can have their own tiling settings, so if you are using separate images (e.g. base colour and Diffuse Overlay) then yes, via the Image Editor dialogue which you can open from the image picker menu on the property. To overlay two maps with different tilings on the same property would, as far as I know, require a custom shader.