Can I stop off on the way? No. These cheap and flexible Britain-Ireland fares are actually cheaper than the normal domestic fares between London and (for example) Chester or Holyhead, so you cannot break your journey, only travel straight through to or from Ireland. Though that doesn't mean you can't wander round Holyhead town centre whilst waiting for your ferry! Of course, if you really want to stop off, nothing stops you buying separate tickets, for example a normal ticket (say) London-Chester, then a Chester-Ireland train+ferry ticket, but this costs a lot more than a London-Ireland through ticket.