Because of the prefix "sci" I always pronounced it "sigh - rah", though through proper English grammar I believe that any vowel followed by a double letter changes it's pronunciation... in which case it should be "sih - rah"??
i think SoldjahBoy's got the right idea here
think science and scimitar, one has a vowel following "sci" but the other has a consonant following "sci" and it changes its pronunciation significantly from "sigh" to "sih", respectively. i, myself, pronounce the "sci" in scirra the same way i would pronounce the "sci" in scimitar, "sih".