I'm trying to write a story about a protective spirit, in the form of a man, coming to the aide of another man who is tormented by demons and evil spirits because of a curse.
There's very few things i know in Irish Mythology.
The first thing that came to mind are the Fair Folk/Fae/Fey of Irish myth, and those come in many forms both wicked and benign. Many Fae will protect a mortal they find interesting enough, or someone they owe a life-debt to. Conversely, many Fae will also torment mortals for sheer amusement. These could be Daoine Sidhe, Pooka, Kelpies, goblins or hobgoblins, or any number of other kiths. Maybe the protective Fae has a personal grudge against one of the wicked spirits, and that's why he saves the mortal.