What Do We Mean By The Word " Gillygoosey"??

The word is used in the story by an old - and very Irish - nun. She is talking about how the novice should be thinking of her trip to Lisieux (which she consistently mispronounces as "Leesoo"). The novice is thinking of her sisters in Dublin, who would at that moment be having tea and leading their uneventful, ordinary lives in the convent. The old nun calls them "gillygooseys" which, as you say, is not a word in any dictionary. She clearly means that they are silly (simple) as geese and it is one of those idiosyncratic and colourful words which one occasionally comes across which are probably made up by a particular person and only used by them. I don't know that there is any proper 'etymology'. Sean O'Faolin has probably heard it used by someone and found it endearing, so introduced it in his story. I suspect that it may be a conflation of the epithets 'silly goose' and 'giddy goose' (meaning roughly the same thing). The old nun clearly has a somewhat endearing characteristic of mispronouncing things (or insisting on her own pronounciation) and such people often have 'made up' words in their vocabulary. An equivalent word (which has a long and illustrious history - http://en.wikipedia.org/wiki/Flibbertigibbet ) is "flibberty-jibbet" (purposely misspelled), which means very much the same as the old nun's "Gillygoosey".