Hi Maya
Every resident in Ireland has a PPS No. Call the tax office to get you the PPS No and you can sort out the tax credit with them over the phone.
No it doesn't have to be done prior to the 31st Oct. Also if you were married in May 2009 you will be able to claim for the remainder of 2009 as well.
BTW when I went through the process of changing the tax bands for my husband and I, I have to say the tax officer I dealth with was extremely helpful. I was very happy and surprised with how easy it was.