The Droste Effect is the name given to an image containing a smaller version of that image which contains therefore a smaller version of that image and so on , to theoretically ad infinitum. The name comes from an early 20th Century Dutch brand of cacao:
The Domhnach Airgid is an early Irish book shrine on display in the National Museum of Ireland. It housed a gospel given, supposedly, by St Patrick to St Mac Cartan:
In the lower left panel we see this specific scene:
At first I was hopeful that this could be a Droste Effect, and a pretty early one – with a mini Domhnach Airgid being passed from Saint to Saint, itself incorporating a mini Domhnach Airgid. It may be in intention but is a blank rectangle… but perhaps the Droste Effect concept was at play. Wikipedia (yes I know) gives the earliest Droste image as 1320 : while this shrine dates from the 8th century the panels were remodelled in the 14th so this may not be a precursor.
but anyway , an interesting little aspect of a beautiful work