I have been to Shanwick many times so know about the place fairly well. I have some photos as well and once I get a chance, I will upload them. It is physically located about 15 miles North of Shannon Airport at a place called Ballygireen. Google location should be this:
Shanwick (a concatenation of Shannon and Prestwick) handles all the Atlantic traffic leaving or entering Europe, till 30 degrees West when Gander takes over. It does that by transcoding pilot originating position data from voice to data so that controllers at Prestwick operate "live" northatlantic taffic data. Not all planes use HF these days. Many have switched to satellite CPDLC (Controller to Pilot Data Link Control - Lufthansa, United, SAS etc) but even these need to stay connected on HF and get voice checked at 30 degrees west.
Shanwick runs a battery of 10KW HF transmitters, derated to 5KW, using NAT-A, B, C, E, F family frequencies. You can listen to Shanwick using online radio on Echolink (if you are a radio ham - there are two HF receivers on Echolink - France and Germany) or on HF receivers you can access here: http://www.globaltuners.com/