Hawthorn, Crataegus monogyna and its relatives, is the May of English usage, the tree so identified with the month that it took its name, and the blossom of the Beltane and May Day festivities.
There is a chronological complication that is usually skated over. Britain moved from the Julian to the Gregorian calendar in 1752, losing eleven days. Hawthorn flowers when it flowers, and in much of England that is now toward the end of May rather than at the beginning; before the change, May Day fell nearer the natural flowering. Old May Day, 12 May, is still observed in a few places and is closer to the blossom. Anyone reconstructing a seasonal festival from the plant needs to know which calendar the tradition was formed in.
The taboo on bringing hawthorn indoors is well attested across England and Ireland: the blossom outside is auspicious, the same blossom on the table is unlucky and in some tellings brings death to the household. A physical explanation is available and is at least suggestive. Hawthorn flowers contain trimethylamine, which is among the first compounds produced by decaying animal tissue. The scent that is pleasant enough in the open air is, in a closed room, recognisably the smell of death. In a period when the dead were laid out at home, the association would have been immediate.
The fairy thorn, a lone hawthorn standing in a field, not to be cut, is a strong and comparatively recent-documented Irish tradition, and it has genuinely diverted road schemes.