Ronan Boyle and the bridge of riddles

Ronan Boyle is the youngest and lowest recruit into the secret garda, an Irish police force that tries to stem and stop the misdeeds of unnumbered magical creatures. Ronan is small, has terrible eyesight, suffers social awkwardness, and has a truly absurd number of allergies--nevertheless Ronan knows he is the only one who can prove his parents were innocent and unjustly imprisoned. To do that, he must go to the land of the leprechauns.