Galls are frequently visited by other wasps for a number of reasons: they contain food and at least one larva which, itself, can be food or a host for a parasite's egg. Some wasps treat the gall as a home for their own larva, which will coexist with the host. These are known as 'inquilines', and are harmless to the creator of the original gall. Different parasitic wasps can target either the original larva or any inquilines which may be present.
Here's the original shot of the egg-laying process from 2006:
|Chalcid wasp ovipositing|
|Oak leaf, showing galls and wasp|
The identification literature on Chalcids has been out of print for some years, and has been very difficult to obtain, so for 7 years I have had to put this down as 'a Chalcid Wasp'. Since we have been in a phase of perma-rain for almost a month, I have had plenty of time to carry out background research, and I finally managed to get a copy of parts a and b of volume 8 of the RES guides:
This was originally published in 1958 (almost half a century ago!) and contains keys and descriptions of 1400+ species of UK Chalcids known at that time, along with a cross-reference showing the target host(s) of each species, if known.
The keys depend on microscopic characters, which always require a specimen and a microscope, so I couldn't key out my specimen from my photographs. The cross-reference, however, showed only three species that specifically target Neuroterus numismalis, so that narrowed things down a bit. Two of the species could quickly be eliminated due to gross features - such as colour - leaving me with Cirrospilus diallus as the only viable option. The description for Cirrospilus diallus fits what can be seen from my photographs and also mentions that it's the most common species of Chalcid in Oak woodlands, so it doesn't seem too unlikely.
Since I haven't keyed out the specimen from first principles, this can't be seen as a definitive identification, but it's not entirely impossible. There is one previous record from Ireland: from county Kildare in 1946. This record was made on the 13th October, which also neatly ties in with the date of my specimen.