Many items to commemorate the diamond jubilee - from postcards to pieces of jewellery - seem to have been adorned with rose, shamrock, thistle and sometimes lotus for India as well. I guess what the painter had in mind was a spray brooch like this (earlier) example, which also includes Welsh leek. Not sure though why he would have felt the need to symbolize Ireland twice, with shamrock in the spray brooch and an additional harp brooch.