The tattoo seen on Ig's arm reads "awake arise or be forever fall'n," which is a direct quote from John Milton's "Paradise Lost" in which Satan falls from Heaven.
The movie is based on the Joe Hill novel Horns.
The character Ig's full name, Ignatius, is only mentioned once during the film. In Latin, it translates to "fiery one" and has the same root as the word "ignite".
While filming in Canada, Daniel Radcliffe accidentally drank nonpotable water from his sink tap in his trailer. Unknown to him, the water was mixed with antifreeze to prevent freezing. Daniel was very sick for three days because of this.
Ig's car's license plate indicates 2036LUK which probably refers to the Bible (Luke 20:36) which reads: "and they can no longer die; for they are like the angels. They are God's children, since they are children of the resurrection."