1.5 hours is a long layover as far as most domestic flights in the US are concerned, have had 30 min connections and made all of them but it was a but scary.
The main one you should be concerned about is the layover where you first arrive in the US. Even if you have cleared immigration in Ireland (I'm not sure if the US Airways flights do or not) you'll still have to collect your bags, bring them through customs and then drop them on a baggage re-check belt. That can take some time.
After that, 90 mins on other sectors should be fine. If you miss a flight you should be re-routed as all your flights are with the one airline. When you mix airlines it's not quite to straight forward.