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I didn't know before, but I see that the Chunnel was drilled largely through chalks from the Cretaceous, and the heavily fractured chalks on the French side caused a challenge to contain the influx of high pressurized water. That certainly does not sound ideal, though I guess the technology then was already available to sort that out.
As for the geology of the bedrock in the Irish sea, it indeed looks like it's older - mostly Triassic, Permian and Carboniferous sand- and mudstones, with some younger igneous intrusions. I'll repeat that I'm not an engineer, but from a first perspective, that sounds nearly ideal compared to drilling through chalks. Chances are high that the rocks will only be lowly permeable.
Furthermore, if you follow the link and look at Figure 2, it looks like the lithologies are continuous underneath the sea. That also sounds a lot more ideal to me than tunneling through a heavily fractured and compacted mess, like the Gotthard Base, which sounds like it was practically packed with geologic challenges.
So at the back of an envelope, chances and cost risks actually look pretty decent for a tunnel underneath the Irish sea.
Regarding the Chunnel, IIRC the problem was that from prior research, the chalk was expected to be continuous and non-fractured on the French side and fractured at the English end, but in practice, the opposite was found to be true.
Regarding the GBT, this was exciting to follow when they built it. From the engineering viepoint, the two biggest challenges expected in advance both proved easier to master:
> high pressure) zone in the south. Before the rest of the tunnel, a test drill was advanced into this area. Amazingly, just a hundred or two metres above the base tunnel's level, the zone became transformed rock which was water-impermeable and easier to drill.
In the north, there was a zone of compressive rock (
I'd wager it'd be far better value for money than a trident replacement
keep to the Fen Causeway
My only other concern would be the risk of bottlenecks in the London area if you are trying to route passengers and freight from Ireland to continental Europe.
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