I am Irish and full time student in Ireland. I was in the US on a J1 visa last Summer (from the end of May to August 2010). I earned wages of $2000, paid $245.51 in federal income tax, paid $61.54 in State income tax, paid $630 on a visa and paid $840 on flights.
Stupidly, I filed a 1040 EZ form on 123 Easy file ( a link of the IRS site) and they said my application had been accepted and that my refund would be $370.
After this was done, I realized that I should have filed a 1040NR form and according to another tax company, I was entitled to $800.
Does anyone have advice? Can I amend/ cancel my application?? Can I get into trouble for this? Is there any way of claiming the extra amount?

You would have had to file from 1040NR or 1040NR-EZ. You would not be eligible for the Making Work pay credit (if that's what the $800 was, are you married?) You can not deduct visa costs and travel expenses. But you will get back all your taxes paid. You need to amend your return. Fill out a correct 1040NR or 1040NR-EZ and send it together with a 1040X.