Such tunnels would be a prime target for IRA Terrorists.
Or jihadists.
Everything, including the Chunnel, is a target for terrorists.
So what?
Would have been. The troubles are over. The only Republicans who are still committed to violence are sad, disorganised wannabes, not the serious, well trained, committed Provisional IRA members who wreaked such havoc 20-40 years ago...
Would the Channel Tunnel have mattered if it had existed in 1941?
Not at all likely given the quality of the remnants, the likes of CIRA, that are around today. Try to keep up.