Hello essnell, thanks for the aid to slowness
A couple of random thoughts. If the foster parent was a chapel keeper, that implies Methodist, or at least protestant. Do you know if the family were RC or protestant? If the former, it might make it less likely that the church was the route to finding foster parents.
Also, given the care the grandmother took to be discreet, I wonder if approaching any church with the conundrum might be a bit too out in the open. Is it possible one of the grandparents knew the foster mother before they went to Ireland? Did she live in the same place the grandparents came from, or were married in, or departed from when going to Ireland? Is she a distant relative or, if the family were well-off, an old family servant (housekeeper, nurse?).
As for records, the Adoption Act 1926 (England/Wales) didn’t come into force until 1927 and, I presume, the little girl would have been 21 by this time. Foster care started to become more formalised in the early 1900s but, again, given the level of discretion you describe, I wonder if this official route would have been too out in the open.
From the information you have, it sounds like a private arrangement with someone the family already knew, or knew of, and whose discretion they could trust.
You made me smile about the census records not obliging with their movements. I also have this challenge with some ancestors with a few being recorded twice in places hundreds of miles apart.
Good luck with your quest,
Flemming.