If you want anything to come over a phone line into your house, whether it be Eircom Broadband, or any other service provider etc, you still need to have Eircom connect/install your phoneline.
Eircom are the only people who will install telephone lines into your house.
You'll have to call them in to get connected. Probably best to let on that you'll be using their services etc. They might work faster if they know they might make money off you.
Then, once you're connected, you'll need to go to whomever you wish for your other services, give them your eircom phone number and account number, and transfer across.