From NCE;

A proposal for a combined bridge and tunnel crossing between Dublin and Holyhead, including two new Irish Sea islands, has been suggested as an alternative to plans for a Scotland to Northern Ireland link.

not unlike the Oresund crossing between Denmark and Sweden.

https://www.newcivilengineer.com/latest/irish-sea-link-proposal-for-bridge-and-tunnel-crossing-via-two-man-made-islands-07-06-2021/