All documentation requirements detailed in the NCTS site [broken link removed], FAQ here.
Registration county is linked to the vehicle's registered owner. So if you want to use your sister's Dublin address, you will need to register the car in her name. She will need to sign a letter giving you authority to register it in her name along with evidence of her name / address (utility bills, bank statements, etc.). Again, all detailed on the application process.
Note the appointment must be requested within 7 days of the vehicle entering the country, if the invoice date is further in the past, you are likely to be asked to provide confirmation of shipping details of when the car was brought into the country (or details of where it was stored if coming from Northern Ireland).