Raggytash.I have used both modes of transport frequently. From my own experience by air from Heathrow,I am fairly sure you do not get airside without going through both security and passport control. On arrival in Ireland my passport has been checked slightly more than it has not been, and when checked in recent years it has often been scanned. As I always carry a passport I have no knowledge of using alternative photo ID.
When travelling by ferry, as you state, you prebook. All names are known and manifested.In in most cases also generating a unique electronic signature, both from vehicle registration and bank card details. Checking can occur in great detail purely from the manifest, if required. Random checks can and do occur both on departure and arrival.
I have experienced them-despite jimmies disbelief, and if your given name is Ahmed or Muhammed, it is likely they will be flagged. For frequent travellers the ferry companies are well aware of your frequency of travel, likewise the security services if you attract their interest. Joe blogs a frequent traveller is of no interest whereas the first time foot passenger Mustapha, who has a huge rucksack, will likely be asked to step aside for a little chat.(very racist profiling, but that is the world we live in)
      I have no problem with checks, they make the world a safer place. In like fashion I have no issue with my bank cards being frozen if I use them thousands of miles from home without informing the bank first.A little inconvenience is a small price to pay.