Well well, three Wordies already listing this as a specifically Joycean word. I just noticed it in Portrait. A stick made of ash, used as either a walking-stick or a cudgel. The oddity is of course 'plant' in this sense.
It turns out however that this is close to the etymological sense of 'plant', which was a sapling, seedling, shoot etc., before being generalized to vegetation of any size or age. The OED says this is now specifically Irish and largely confined to 'ash-plant'.