I'm not sure about this, but I suspect the fact that the Stena Line doesn't connect well with Britrail (at least at Holyhead) may have something to do with the fact that the BritRail + Ireland pass comes with passage on the Stena Line. If you've got a competitive disadvantage (as Stena appears to at Holyhead, since Irish Ferries has the time-slot just after trains arrive), you promote yourself by "bundling" the package to make yourself more attractive to customers. I think that's exactly what Stena is doing here. Heck, it got us to buy the package, and only use their ferry in one direction.