Don't take books like Joyce as holy writ. Placenames are subject to subtle changes that disguise their origins, and replace them with false leads. It's very common in English, and you can be sure that Irish is not immune. A good example is the "sea" ending in places such as Chelsea and Battersea, which has nothing to do with the ocean, but is derived from a word for "landing place". His history is distorted by nationalism and Celticism, and he seems determined to deny "alien" influences whenever possible. There's a lot of good scholarship gone on since then.