Yes. Many carriers in the US offer prepaid mobile plans, though it will be more expensive than some offerings in Europe.
This article offers a decent summary of the main options, especially T-Mobile and AT&T and the numerous providers who resell their services. You can go to one of their stores in Los Angeles to get a SIM card and get setup. Or if you'd prefer to not spend part of your trip waiting at a cell phone shop, there are vendors, with varying prices, who will ship a SIM card to you in Ireland for you to bring with you. The plethora of options, virtual network operators, and resellers makes price comparisons obnoxiously difficult, but you should be able to pick up and get setup with a SIM card without too much trouble.