Not exactly Celtic, but Tolkien and fantasy seem to fit right in if you ask me. This is a couple of weeks after the opening so hopefully it will have settled down some and won't be sold out.
Link for tickets (shortened): http://goo.gl/Tbb8W
If the link doesn't work, go to thestoryoftexas.com and navigate to the IMAX schedule/showtimes to buy tickets. The link above is for tickets to the 12:30pm showing.