Irish themes often end up cloying or playing to stereotype. This movie, with its "mystical" Celtish theme and its message of Irish pluck could easily have fall into either trap.
But the kids -- and the horse -- save the day. Great actors, those boys. Great choreography with a beautiful horse. Together, these elements make the story remarkably engaging -- keep the story itself from devolving into bathos, and make it impossible for you not to suspend disbelief and root for the horse! Great directing to bring this all together.
Example of how such a fantastical story is presented with some subtlety: in one almost inconsequential shot, the two boys are walking down a crowded Dublin sidewalk, leading the horse. A couple of kids pass them, and throw unprovoked verbal abuse at the boys, disparaging them for being travellers. The older boy quickly spits back a retort or two in kind and then returns seamlessly to his ongoing conversation with his brother -- but doesn't break his stride and doesn't show any signs that the abuse has even registered with him. It is as once heart-rending (what does it say about his life so far that such an attack barely registers, it must be so common), and a testament to the boy's courage.
In that one brief scene, the movie shows the depth of character the kid will draw on throughout the movie, and you can't help but root for him! And the acting makes you feel like this is really who the boy is, not that he's an actor playing a part. (It was such a better scene than the heavy-handed scenes with the requisite bad cop.)