I finally received my P45 from my old employer (like 7 week safter starting my new job!) can I just send it straight to the Revenue or do I have to give it to my new employer?
You should give the P45 to your new employer asap to avoid paying emergency tax. Just keep a copy for your own records in case it goes astray. Good luck in your new job!
You give it to your new employer. Your previous employer will have sent part 1 to the Revenue Commissioners already. He should have given you parts 2,3 and 4.