Do not read my review if you are planning on see the film.
It amazes me how a film can be so misunderstood. Gleeson is suffering from a severe pyscotic episode and has suffered from depression all his life. Look at the death masks, and miniature hanging by the neck figures in his cottage. There are even African death masks on the walls.
Gleeson, Farrell, the priest and Farrells sister even use the words depressed and despair on a number of occasions when referring to Gleeson with 30 minutes of the beginning.
Farrells character is not the brightest, can not understand depression or human nature in general so he connects with animals more than humans.