People have no idea how big these cables really are. When they're up on pylons they don't look a whole lot bigger than the cable going into the back of your washing machine.
We've had similar protests here where vitally needed overhead 220kV lines were held up around Cork Harbour for similar reasons. The ESB didn't want to run them subsea under the harbour as there was a very serious risk of anchor damage due to the busy nature of the harbour and estuary. There was also a very signifigant increase in cost involved even for this relatively short crossing.
400kV's been used here for quite a while but only on 2 lines that connect a very large coal-fired powerstation on the west coast to the greater Dublin area on the east coast. However, they terminate some 60 miles outside Dublin and feed into the local 220kV and 110kV infrastructure.
The original 1920s grid was 110kV and 220kV was introduced as demand went up. 400kV only arrived in the 1970s.
