What is this (possible) fossil from the triassic/jurassic boundary?
Its definitely a flowering plant which evolved in the early Mesozoic Era. Also, Pangaea was beginning to break up, but not fully broken up so climate would be than likely be Continental-Maritime allowing for dry weather and water cycle contributions from the surrounding body of water as it broke apart. Also, this land was positioned a little closer to the equator at that time and if found in Ireland, it would have been 'attached' to northwest Africa and the east coast of North America to the west making it possible from a climate and geographical situation. Have a look at this video of Continental Drift movements of parts of the world at 200 million yrs ago in the roughly similar time period you suggest: view at 3:47 for 200 mya and 4:03 to view the recognizable formation of the British Isles. Enjoy! https://www.youtube.com/watch?v=UwWWuttntio