Meadowsweet was the first plant from which salicylic acid, or aspirin, was synthesized. It has traditionally been used for digestion as well as inflammation and pain, unlike plain aspirin which is hard on the stomach. Perhaps people were buried with Meadowsweet for a pain free experience in the afterlife.
Sources:
John Lust, The Herb Book (New York: Bantam Books, 1974).
Patricia Monaghan, The Encyclopedia of Celtic Mythology and Folklore (New York: Checkmark Books, 2008).
The Order of Bards, Ovates and Druids, “Druid Plant Lore,” http://www.druidry.org/druid-way/teaching-and-practice/druid-plant-lore
The White Goddess, “Meadowsweet,” http://www.thewhitegoddess.co.uk/herborium/meadowsweet.asp
Barbara G. Walker, The Woman’s Dictionary of Symbols and Sacred Objects (San Francisco: Harper and Row, 1988).