I'll have to look up A Star called Henry. If you haven't seen Morgan LLewellyn's 1916, that is of course fiction but having read Rebels and Max Caulfield's The Easter Rebellion, I can see where a lot of teh stories came from. (Of course she probably went to the primary sources cited in those books)
I think the name was probably pronounced as if it were the more conventinal O'Reilly and my reason for believing this is via James Joyce-- a Leprechaun probably knows that the hero of Finnegans Wake was one Humphrey C. Earwicker. At least one of the reasons that Joyce chose that name is that ear-wicker translates into French as pers-orielle and sure enough one of the characters in the Wake eventually sings the ballad of Pearse and O'Rahilly (names of course mangled as can only happen in a dream) Circumstantial evidence only, and maybe misleading, but I'll believe it till something better comes along. There's a picture of him somewhere in Rebels-- he didn't look at all like I expected him to! (neither did Tom Clarke)