It is not a novel idea to see if rhizobia can also be beneficial to non-legumes – a Google search found that several other scientists have been studying how this could work for several years. However, the other scientists have been studying smaller portions of the biochemical process behind how plant species react to compounds produced from rhizobia (from what I could find without thoroughly looking int it). The way the Irish girls' research is unique is that they chose to test two important cereal crops used in worldwide food production, apply a rhizobia coating directly to the seeds, and measured germination and plant yield at a large, replicated scale (the girls planted 9,500 seeds, both in a greenhouse environment and outside in the field!). None of the seeds or plants experienced negative effects from the rhizobia – germination of the seeds increased by 50%, and yield increased from an average of 30% up to 70%. This led to the girl's conclusion that their research could have major agricultural implications. Grass species produce flavonoid compounds just like legumes, so if rhizobia are detecting these and producing the same lipochitooligosaccharides (LCOs) that they produce in legumes (just without the nodule formation), this could be helping increase seed germination, as past research has found.
The girls' research could allow farmers to apply rhizobia or bacteria to their seeds as a type of fertilizer, and then they could plant seeds earlier in the season, or in wetter conditions that in the past have led to seed rot, since seeds will be less likely to rot if they germinate twice as fast. With increased yield, more grains could be produced per plant, and seedlings could grow faster to avoid being as susceptible to disease and harmful conditions near the ground when they are short and young.
The girls have already received a patent to incorporate their research in the malting step used for industrial brewing, and several research groups are interested in further discussing potentials of this research.
Here's a National Geographic summary of the girls' research:
Here are the details of the project from the Google Science Fair website: