Try to arrange accommodation before you travel, or at least establish where you will be staying for your first week in the US. The standard J1 programme fee includes one night's accommodation, but after that you're on your own. If you're applying for a job in advance, ask your employer for assistance with finding accommodation. Those who regularly employ students may provide accommodation at a reduced rate, while most other employers will usually do their best to point you in the right direction.
Don't limit yourself to Irish enclaves like Boston or New York.
Jobs are filled very quickly in these cities and the cost of living is higher than elsewhere. Off the beaten track your nationality is a real selling point.
Under US tax law, you are obliged to pay income tax on your earnings. The good news is that you can claim that tax back at the end of the tax year, using the (rather complicated) form available from the US embassy.
The US is a credit-card culture and it can be difficult to book accommodation in advance, book tickets by phone or hire a car without one. A credit card will also ensure that you have some form of back-up in the event of an emergency. Your local bank official will probably laugh like a drain if you attempt to apply for a credit card on your own behalf but banks will issue a credit card as an addition to a parent's card or with a parent as guarantor.
Don't bring large amounts of cash with you. Keep most of your money in the form of travellers' cheques, which US businesses accept with passport identification and which most of them will happily cash for you.
Always carry ID, both as identification when cashing traveller's cheques and, if you're over 21, to enable you to buy alcohol. Generally, only a passport is adequate for cashing cheques or entering bars; a student card may not be accepted as proof of age or identity. Photocopy your passport details - identity pages and visa stamp - before you leave the country; you can carry the photocopy as ID and leave the passport somewhere safe. The photocopy will also help you if your passport is lost or stolen.
Buy a decent guidebook. With a little luck you'll earn enough to enable you to travel around for a few weeks when you finish work, but at the very least a good guidebook will include useful advice on accommodation, cheap dining, hospitals and contact numbers in the event of an emergency. Best of the bunch are the Rough Guides and Lonely Planet guides. The Let's Go guides are rubbish and are only for those who think culture is something you find at the bottom of a yoghurt carton.