in basic speak.... cat 6 or cat 5 is used as a network.
What you need to do is ask yourself
(a) where you are going to plug in your phones, and get phone sockets (RJ11) there, connected to the main entry point for eircom or bt.
(b) where are you going to have your computers, media centres, media devices, ip cameras, audio points etc, and have data sockets there, connected to where your router is going to be (increasingly out of sight nowadays, and more often than not beside the phone entry point.) Data points are then networked to this central point with CAT5 or CAT6 cable.
hope this helps