Would reducing lyrica (pregabalin) from 600mg each day to 450mg each day cause irritability. many thanks?

It is possible: If this medication is being used to stabilize your mood, it is possible that lowering the dosage may cause an exacerbation of symptoms which could present as irritability. On the other hand the lower dosage could be right for you or you may not need the medication at all. Changing the dosage should be under your doctor's supervision.