If you obtained a foreign nursing degree, you would need to apply to the Council for Graduates of Foreign Nursing Schools (CGFNS) and wait to see if you would be eligible to sit for the NCLEX-RN examination. That is just to get a license.
Step 2 would be to have your Irish transcript evaluated by a credentials evaluation service which would 'translate' the grades you received there into an American A, B, C, D, F and 4.0 gpa standard. They would also indicate what American degree your Irish degree was equivalent too (an associates degree, a hospital training program, etc).
After you have an American RN license and had your foreign degree evaluated, you would then apply to a US school.
Here is a link to the steps to become a nurse in Ireland from the Irish Nursing Board: http://www.nursingcareers.ie/Becoming_a_Nurse.HTML