Access between Scotland and Ireland is currently limited to travel by air or sea.
It has been like this for almost a century since air travel was invented.

Many other countries have tunnel links of substantially greater distance. 
The concept is nothing new. It's just that Britain has fallen behind the rest-of-the-world with it's once world-leading, but now antiquated infrastructure.
Yet the world is changing fast and efficient infrastructure is now a vital component of economic development.

This Concept Proposal graphically illustrates how a new land route involving a tunnel and two bridges can be constructed, linking Scotland and Ireland via the Mull of Kintyre and Antrim route.

The main crossing point for freight and passengers in Northern Ireland is currently the Cairnryan to Larne ferry route. This is susceptible to disruption due to weather conditions.
A quick check of ferry charges reveals that travelling by this route is an expensive process. When competition is introduced, such as Channel Tunnel alternative on the Dover route, charges are significantly lower.

The Concept Graphics below illustrate how this can be achieved.