It may have been, since the family was so deeply rooted to their Irish culture and rituals, that the practice of throwing water is directly related to an ancient tradition for the protection of the path of the deceased onto another world. Water, being an element of purification and cleansing is blessed by the priest and throwing as a way to bless and clear the path as they transition.
There has been much speculation since Frank McCourt's book Angela's Ashes was adapted for film regarding the scene in which water is poured in front of a funeral procession. MCourt's book does not include the scene, so no explanation is provided there, and the filmmakers do not seem to have fully understood the purpose of the "tradition." Research into funeral traditions, especially those born of Catholicism in general and Irish Catholic history in particular, offer an explanation. To some Catholics, crossing water is an integral part of a soul's crossing into the next life, with the water preventing its return. In other words, the water guarantees a one-way journey for the spirit. [See "Funeral Feasts and Processions," http://www.sacred-texts.com/etc/fcod/fcod09.htm]. The best explanation, however, comes from the film's co-producer, David Wimbury, whose explanation was provided in a Los Angeles Times column:
". . .the picture's co-producer, David Wimbury of Dirty Hands Productions. Their London-based Irish prop man had suggested the scene based on a practice of the past. The water used to wash the corpse before placement in the coffin was traditionally kept to be thrown in front of the hooves of the horse drawing the funeral carriage. Later, this developed into the symbolic act of neighbors and family throwing buckets of water as a mark of respect for the dead."
McCourt's family history is obviously heavily influenced by Catholicism, and traditions common to such orthodox practices as those described in the lower-income Irish Catholic community at the core of Angela's Ashes allow for creative liberties such as those employed in the movie version of the book.
Apparently it had become an Irish tradition to hold back the water used to bathe the corpse, to later throw it in front of the hearse as the funeral procession rolled by. Why? Who knows, but thats the tradition and thats why they do it in the book. You can hear director Alan Parker say so during the directors commentary if you own the DVD.