To avoid the deep, explosives-filled trench that WW2 Ordnance was dumped in. Stranraer, of course is/was a port - but with Portpatrick being the closest point of land, the trench is off this.
The real hack here is that the ordnance is on the bottom of the sea. The tunnel would be
under the bottom of the sea. More challenging is that Stranraer/Portpatrick don't lend themselves to onwards connections, and the depth of the trench is such that the gradients would be extremely unfavourable or the tunnel portals so far inland that you start undermining the theoretical benefit of the shorter crossing.
The Galloway-Belfast tunnel only really makes sense if you want the whole thing to have a Union Flag on top of it. The biggest flow for such a fixed link is always going to be London-Dublin.
I worked on the case for building a tunnel between Dublin and Holyhead many decades ago, the company I worked at had some of Europe's top tunnelling experts. The numbers stacked up if you assumed virtually all England to Ireland and Northern Ireland air passengers and all road freight moved to the tunnel. What was not included in the numbers was the cost of upgrading and electrifying the rail line from Crewe to Holyhead and building a new rail link into Dublin. In those days without HS2 the reality would be that London to Dublin would still be around 4hrs and so it would fail to capture a lot of the air market especially as Ryanair was revolutionising air travel then.
That's actually better than I expected! It's within the scope of HS2 improving the rail journey time, and whacking air travel with the environmental stick, to make feasible. Not that the engineering challenges are insignificant, but engineering challenges are generally solvable if someone wants them to be solved.
I'd assume that an Irish Sea car carrier (An Shuttle?) would be desirable to take road freight and car traffic off of the ferries. Ironically, you'd probably then need to upgrade the A55 to take the lorries that would have gone to the ferry ports, and similar upgrades to the Irish road network.
On a similar note, a full North Wales high speed line probably isn't viable given the terrain and population distribution. But an HS2 spur from Crewe to somewhere on the west side of the Dee estuary might work. With luck we can fob off the electrification from this point to the tunnel terminal on another budget. If we're doing this, we're electrifying all the main lines anyway, right?
The wider strategic position gets ignored by these proposals. While Irish politicians get very annoyed when Britain diverges from Ireland, Ireland's main strategic goal for over a century is diverging from Britain. A tunnel would go against the goal of diversifying trade. It's not something any Irish government would stump up €10-20bn to pay half of.
It would probably be easier to make the case if Ireland and the UK were inside the same political unit.... not that making it easier to build 305mm to the foot scale model trains is a good reason for doing that!
