I finally found it:

One person was appointed to guard the crops on St. Michael's
Day and to make a circuit of the township on St. Michael's Night.
Women gathered wild carrots a few days before the festival on
Domhnach Curran, Carrot Sunday. Recitations accompanied the
pulling out of the carrots, either by hand or by special mattock. It
was considered good luck to find a forked carrot, a symbol of
fertility. The festivities also included the sacrifice of an
unblemished lamb and the special St. Michael's Cake baked on a
lambskin and moistened with sheep's milk. The cake, known as
the struan, was baked on a large, flat stone; it had to remain
whole during baking or evil would befall the family. It was then
taken to church to be blessed.