Well, if you look at the way it's spelt (spelled), you would see it as "Tear-ee-an."
Tirian. It would only be the way you say it if it was spelt like "Trinian."
I'd make the vowel sounds and stresses exactly the same as in "Miriam", and just adjust the consonants a little. That's how I hear it in my head, anyway. Short i as in "pip".