Your route is totally valid for a 3-continent RTW; adding Australia makes it a 4-continent RTW and increases your price (and her commission?) big time. Suggest you call the BA RTW desk in London, (someone have the phone number?) explain your itinerary, and they will help you all the way to ticketing.

You should probably note that the cost of DONE3s originating in Ireland are higher than almost anywhere else - over 600€ more than originating in the UK, 1500€ more than originating in any of the continental Euro countries (and even greater savings in some Eastern European countries.) It might be well worth your time to hop over to some other country for purchasing and ticketing - see a number of threads on this board for related topics, and look at the sticky at the top of the board for links to pricing.