I've lived in Ireland and the bit about "thin places" is very true. I've experienced them:
Barbara Brown Taylor, one of the best preachers in the Episcopal Church today, once gave a sermon on the Transfiguration in which she spoke of "thin places." She had just returned from Ireland, and it seems the Irish have a notion of something they call "thin places." Thin places are the soft, porous, permeable places where the veil between this world and the next is so sheer that it is easy to see through them and perhaps get a glimpse of the power of Jesus' life penetrating into our lives. I remember the noted biblical scholar, Marcus Borg, also referring that to Celtic, Irish spirituality as he wrote about "thin places, opening the heart."-- from a sermon by The Reverend Elton O. Smith, Jr.
I am suggesting that the disciples with Jesus on the mountain had encountered a thin place between this world and the future that Jesus was revealing to them. They suddenly recognized Jesus for who he really was, for what he represented both in this life and the one to come. And as the light of Jesus' presence shone on them, they were transfigured that day, to see the next steps in their lives in terms of their potential.
Think about it: in this transfiguration story, Jesus does not focus on himself, but on those who were with him. The story is as much about them as it is about him. It is here that they see the connection of their lives to the stories they grew up with about Moses and Elijah. Now their eyes were finally opened to the direction Moses and Elijah had pointed, the direction that Jesus was now revealing.