On another note, what is the deal for a prospective tunnel
connection to Ireland?

(Yeah, right.)

Connecting a network with standard 1435 mm track gauge to another
network with 1600 mm wide track gauge?

If that was the only problem, it wouldn't be a problem.

And then?

Most conveniently, what they do with some trains at the French-
Spanish border: slide the wheels along the axles to fit the other
gauge. Other solutions include mixed-gauge track, bogie changing,
and (of course) having the passengers change trains.
--
Mark Brader | "For the stronger we our houses do build,
Toronto | The less chance we have of being killed."
| -- William McGonagall, "The Tay Bridge Disaster"