Seriously? Tunnels and overpasses shoehorned into Dublin city Centre?Ah, the either/or fallacy! Firstly road space isn't a constant. You can increase capacity by smarter traffic signalling systems, tunnels or overpasses. Second, you can vastly increase public transport capacity by going underground. It is the green lobby that constantly demonises motorists rather than advocating for a solution that works for both cars and public transport users.
It would cost tens of billions of Euro in construction, take decades and cause billions in disruption.
We could have drones or helicopters picking up cars and flying them into the city centre. That would be cheaper and more commercially viable.
I suggest that if we are boring tunnels we'd be better off running trains through them. That way we wouldn't have new roads popping out of the ground in what are already congested city streets. I travel 10Km from one side of Dublin to the other each day. Even with the fantastic lockdown traffic it is only 5 minutes slower on the bike. Dublin is far more cycle friendly and safer than it was 20 years ago. That means more people cycle so there is more room for those who drive.