Nit-picking about the Celtic Otherworld of 1666

If you set a story in a Celtic Otherworld which is co-located with London, your otherworldly Celts are not all that likely to speak Irish; a lost eastern dialect of Welsh is more probable.

If you set a story in 1666, and your viewpoint character does not have access to time-travel, he probably would not be familiar with the concepts of telegraphing, or oxygen.

Just saying, like.