Until now, I’ve been under the assumption that these boundaries were all imported from some external dataset. However, the documented process for mapping townlands sounds exactly like the process for manually mapping something in OHM based on old maps. Anyone could carry out this same process in OHM, but it wouldn’t be as straightforward as loading a shapefile and massaging the tags.
If the LEAs and historic census boundaries were mapped in similar fashion, then unfortunately there’s no silver bullet. You’d have to ask for permission to transfer others’ contributions under CC0. In the past, some have promoted the idea of “per-feature licensing”, so that you’d transfer whatever you want but tag each individual feature with license=ODbL. However, I personally don’t find that to be a sound or sustainable approach and don’t recommend it for these boundaries.
As I understand it, at least some townlands remain relevant in everyday life in rural Ireland, so I certainly wouldn’t push for their removal from OSM. But frankly, I’m amazed that the Irish community has gotten away with mapping such boundaries as Pembroke Township [1863–1898], Drumgath Electoral Division [1911–1973?], and Pembroke LEA [1998] in OSM, given the sensitivities around defunct features such as historic railways. If someone were to map an analogous boundary in my country in OSM, they’d get called out for it or summarily reverted. This practice is detrimental to both projects, getting in the way of OSM mappers while depriving OHM mappers of useful points of reference.
Fortunately, it seems that at least some of the mappers involved with these initiatives are still actively contributing to OSM. Perhaps we can reach out to them to figure out where to go from here?