1. Data link is used for the pre-departure ATC clearance only, so no log-on as such, just a request sent into the ether (via the datalink menu)..... Taxi or push back is done the old fashioned way by VHF voice.
2. After takeoff the request for Oceanic clearance is again via Datalink not earlier than 90min before oceanic entry (which ex-LHR usually means getting the request done during the climb).
3. The actual Log-on to CPDLC with EGGX is not done until a maximum of 45 minutes prior to Oceanic entry, so you're usually over Ireland.