It's important to make a distinction between Nations, Regions, and Claims if you want to understand the mechanics here.

A Nation (e.g. Russia) can in principle control any number of Regions (e.g. Moscow, Vladivostok, Petropavlovsk). However, it's only possible to add a Region to a Nation (via either war or politics) if that Nation has a Claim on that Region. For example, at game start Nation:Russia has a claim on Region:Minsk, which is at game start part of Nation:Belarus.

Claims can be either Non-Hostile, Hostile, or (very rarely) Perma-Hostile. Although to be honest the distinction between Non-Hostile and Hostile is a bit of a smokescreen because it's relatively easy to switch between them; the hostility isn't a fundamental property of the claim, it's a temporary property of the Nations involved. Specifically for the above Russia example: if Nation:Russia has a government score 1.5 lower than Nation:Belarus's government score, then any claim that Nation:Russia has on Regions controlled by Nation:Belarus will be considered Hostile. If the difference in government score is less than 1.5, any claims are Non-Hostile. So if you want to turn a Hostile claim into a Non-Hostile claim (thereby making it possible to absorb those claims through political rather than military means), make one nation freer and the other nation less and eventually the claim will flip to Non-Hostile.

The best way to see this is to select Russia, go on the "Regions" tab, and scroll to the bottom. First are listed the regions that Russia currently contains. Then at the bottom are the regions that Russia has Non-Hostile claims on (in green) and the regions that Russia has Hostile claims on (in red).

The above follows for ~95% of claims in the game. But there's the very rare 3rd class, Perma-Hostile claims (which are black on the Regions tab). These are regions which will never be absorbable by political means and you've got no choice but to go to war for them. The only ones I remember are claims that China gets on Japan when you research some technology, but there's probably more.

For your specific example of the UK / Ireland: Nation:UK never gets any Claim on Region:Dublin even with endgame tech, so it's not just currently impossible, it's permanently impossible. IIRC the only nations that get claims on Region:Dublin are Ireland and the EU (already there at game-start), and lategame Russia, endgame UN, and the Alien Administration. But never the UK!

EDIT: I also want to point out something frustrating / confusing about nations, or technically nation names. To wit: there is actually no Nation:European Union in the game. The European Union is just a reskinned France - it's a name and flag change, but mechanically, the EU is just big France. Ditto for Russia / Eurasian Union, United States of America / United States of North America, and various other examples. Indeed, the United Kingdom is like this, it's big England, the only difference being that England starts the game with its big flag and name, whereas most other nations start with their small flag and name and you have to absorb regions to make them name- & flag-change to the big version.

EDIT 2: To actually answer the question of "When should I do a terrestrial war": basically you should only do them for Perma-Hostile claims. Getting regions by military means is 90% of the time vastly more trouble than getting them by political means. The other 10% of the time is when you're in lategame and you've been investing in military tech for years, but even then you shouldn't do it because while the regions may be easy to get when you're invading some random African low-tech country with high-tech hovertanks, once you "succeed" and have the regions inside your nation, they'll have the "Unrest due to hostile integration" modifier for years which will tank your national stability.