As you are going over for a trial period, they may not be transferring your job to the US and may be using some form of secondment. From a tax perspective, if you remain an Irish company employee until you decide to make the move permanent, they may be tax equalizing you to your Irish earnings. Your living and/ or working location in the US will impact the level of tax that you pay due to different state income tax levels. There is probably also an impact on the type of work Visa that you would be getting.
If you have other income in Ireland, e.g. rental / investment etc. I would strongly recommend that you request that your employer engage a tax advisor to work through your specific situation. It is important to remember that tax is a personal responsibility, at the end of the day the revenue authorities will chase you for unpaid income taxes not your employer.