I have the small rib cage/big boobs ratio issue - I am a 28DD. Because of my weird sizing I have to buy online, and I use breakoutbras.com. Their CS is REALLY helpful. I have a Freya bras. They are these:
http://www.breakoutbras.com/product/...Underwire-Bras
http://www.breakoutbras.com/product/...Underwire-Bras
They're great and were picked for me based on my "less mass on top" issue (i.e. really floppy boobs). They have more side coverage than I was used to (I used to wear 32B bras - I was SO in the wrong size!) but they do not irritate at all so I'd say if it's irritating after a couple of wears, it won't get better.