It is likely that this brooch could have been a gift to The Queen recently, if it was inherited either from Queen Mary of the Queen Mother, it would no doubt have been seen sooner in connection with Northern Ireland engagements.
It is possible that this brooch was a gift to The Queen to mark her Diamond Jubilee, perhaps from the Northern Ireland Assembly or a similar institution.
Officially no information has been released about the piece, whilst it was announced that The Queen wore a blue and white outfit by Angela Kelly, it was only mentioned that the outfit was "complimented by a diamond shamrock brooch".
Again yet another of The Queen's many many brooches that we know nothing about.