There has never been a hard border between Northern Ireland and the Republic of Ireland.

Until 1922 the United Kingdom was the United Kingdom of Great Britain and Ireland, and as such there was no border.

In 1922 the Irish Free State seceded from the United Kingdom. The UK government judged that it was impractical to patrol a 300 mile long land border (something which they had little experience with, given Britain shares no other land borders) and proposed to their Irish counterparts that they continue with the status quo. In order for this to be practical, both countries would have to agree to enforce each other's immigration policies; agreement on this was reached in early 1923 and relevant legislation was passed in both countries to this effect in 1925.

This is the birth of what would become known today as the Common Travel Area. This effectively amounted to a shared immigration policy, since each country effectively agreed to admit any individual who had been admitted to the other country. It is worth noting, however, that immigration policy was significantly simpler and less contentious than it is today.

The CTA was suspended at the outbreak of war in 1939 (Ireland was neutral in WW2, although they did enter into war planning scenarios with the UK for a joint defence in the circumstance of a German invasion of Ireland). It is interesting to note that even in this case the UK judged it impossible to establish a border between the RoI and NI; border checks were instead implemented between the islands of Ireland and Great Britain.

These restrictions lasted until 1952, delayed by the failure of Ireland and the UK to agree a "similar immigration policy", reflecting that control of immigration - rather than restriction on smuggling and security - were becoming more prevalent in a post war world.

In short, there has never been a hard border between the Republic of Ireland and Northern Ireland; there has, as recently as 1952, been a hard border between the islands of Ireland and Great Britain.

Will have to disagree with the original answer by u/vinylrmulator

Only insofar as to point out that by the 1970’s travel across the border did become somewhat more problematic.

This was obviously due to The Troubles.

Several small roads on the British side were torn up by diggers, preventing free travel between the two nations and limiting the routes available and while not always strictly checked, there did exist border checks between the North and the South.

Passports would be checked on the Dublin to Belfast train (when it was running- destroying the line was a favoured tactic of paramilitary groups) and a highly lucrative series of smuggling cartels began operating around the border region.

Certain parts of the border were much harder than others. Certainly the region around South Armagh was a very VERY hard border due to the British Armies inability to patrol the region safely.

The hard border as it was led to some charged situations between the two nations. On May 5th 1976 two members of the SAS were arrested at a Garda-Irish Army checkpoint at Cornamucklagh, 200 yards south of the Irish border.

It transpires they crossed the border to get to Flagstaff Hill (in the south) to pick up two other members of the SAS who had been carrying out covert surveillance of two border crossings. This led to four more members of the SAS to drive the same route a few hours later looking for the missing men and getting arrested at the same checkpoint.

All 8 soldiers were later tried in March 1977 and fined IR£100 each for possessing arms and ammunition without a license.

The ‘hardness’ of the border and its relevance was best summed up by Captain Nick Lewis of the Coldstream guards who said in 1996 the border was ‘a brick wall’ in regards to military operations.

It was for security purposes that both sides enforced a ‘hard border’ (aka a border blocked by physical impediments and deployment of security officials) from the 1970’s until the 90’s.

It was more eagerly enforced on the British side although it was noticed with some grim irony, that the moment BSE broke out the Irish enforced their side much more stridently (with subsequent complaints by smugglers).

The perception and significance of the idea OF a border also had a deeply profound resonance upon the entire conflict. In 1991 one RUC Officer commented that he no longer checked under his car for car bombs as there was a belief that the conflict was winding down- ‘What’s the point of fighting to get rid of the border, when all the borders come down in ‘92?’ (Referring to the EU open borders agreement of that year).

This then is the ‘hard border’ between the two that is referred to.

Sources: Harndrn, Toby ‘Bandit Country: The IRA & South Armagh’

A much more detailed account of the causes of The Troubles can be found here.

Edit: tagged in original answer if they wished to rebut; people don’t argue with one another on this sub; its historians so we may disagree over the sources we use but it’s never personal.

I saw the original answer as an excellent coverage of the legal status of the idea of a hard border; all I did was add the physical history of events.