It is not necessary to have a job waiting for you in the U.S. What is necessary is you meet the Affidavit of Support requirements (read about Form I-864).
During the visa process you'll be asked to provide at U.S. address. This doesn't have to be where you live, it does have to be where you can receive mail. A relative or friend's address is acceptable.
For the DCF process, once you're approved for a visa (in your case in Dublin), said visa is valid for six months. And yes, once you enter the U.S. you'll receive your green card (aka permanent residency) in the mail.
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