It was unpopular among Brexiteers because it prevented the UK from negotiating its own trade deals which might differ with the rules contained in EU trade deals. It was unpopular with Remainers because it was so obviously inferior to full EU membership as the UK would become subject to rules it had no direct say in formulating. It was hated by the DUP because it did not achieve their unstated objective of ring fencing "Ulster" (in reality 6 of the 9 counties of Ulster) from the rest of Ireland.

It was also a major concession by the EU because no other third country has been given such unfettered access to EU markets without paying a heavy price: in the case of Norway, something approaching the UK's current, and much hated, net contribution to the EU on a per capita basis. Although everyone agreed it would be temporary, no one believed there was any other way of keeping both the Irish border and the Irish Sea open. Neither the Norway+ or Canada+++ free trade deals mooted by the UK as a basis for their future relationship with the EU would have kept the Irish Border open.

The Irish government, supported by the EU, pursued a hard line on retaining the backstop within the agreement despite a lot of pressure from the UK to back down. Doing so would have meant accepting that, sooner or later, border controls would be required once the transitional arrangements, and any extension thereof, had expired to be replaced by some form of trade deal, however extensive. The risk of that happening would also have been a powerful lever for the UK to force further concessions from the EU in the future - effectively unfettered access to EU markets indefinitely - without having to pay Norway like contributions to the EU budget. Effectively the UK objective of "having its cake and eating it" would have been achieved.

But there were also powerful domestic factors forcing the Irish government's hand in all of this. The centenary of the Irish Civil War 1922/3 is coming up, a war fought over the Anglo Irish Treaty which created the Irish border in the first place, and which has continued to shape Irish politics ever since with the two main political Parties - Fine Gael and Fianna Fail representing opposing sides in that war. One of the great achievements of Irish politics has been the relatively peaceful resolution of that civil war in the south, and, much more recently, the resolution of that conflict in the North through the Belfast Good Friday Agreement of 1998 (GFA) which guaranteed "equality of esteem" to both the Nationalist and Unionist traditions there.

The GFA was successful largely because both Ireland and the UK were members of the EU and committed to "an ever closer union" which has gradually dissolved the hard edges of British rule in Northern Ireland, enabled some devolution of powers to N. Ireland, and reduced the border to some long abandoned check-points and military fortifications. It did so while scarcely having to mention the EU because A.50 of the Lisbon Treaty didn't exist then, and no one gave a thought to the possibility of either Ireland or the UK ever leaving the Union.

The DUP, for its part, opposed and resisted the GFA, and only very reluctantly came on board after the 2006 St. Andrews Agreement which enabled Ian Paisley to become First Minister in a devolved administration which also included Martin McGuinness of Sinn Fein as deputy First Minister. The pair developed an excellent working relationship, much to the chagrin of many of their supporters, and became known as the "Chuckle Brothers" for their jovial and harmonious relationship in public.

But that was then and this is now. Personal and political relationships gradually deteriorated under Ian Paisley's successor, Peter Robinson, and then fell into free fall under his successor Arlene Foster, to the point where Martin McGuinness pulled Sinn Fein out of the devolved administration shortly before his death in 2017, an administration that has never been revived. Relationships deteriorated even further due to Arlene Foster's responsibility for the Renewable Heat Initiative Scandal and then the DUP's support for Brexit, even after it was decisively defeated in the referendum in Northern Ireland by 56-44%.

The DUP is a very right wing hard line (British) Nationalist party which would have been ideologically well disposed to Brexit in any case, but in this situation Brexit also represented an opportunity to get one over their opponents Sinn Fein, who campaigned for Remain. It was an opportunity to emphasise N. Ireland's distinct identity from the rest of Ireland, and emphasize its closer alignment with Great Britain.

The DUP claims any customs controls in the Irish sea would undermine the Constitutional integrity of the Union while seeing no difficulty with greater (virtual or otherwise) controls across the Irish border. But if some greater controls in the Irish Sea (in additional to current animal and food safety checks) represent a change to N. Ireland's constitutional status - which under the GFA requires majority agreement - how much greater a Constitutional change is Brexit itself, which has been explicitly rejected by a large majority of the people of N. Ireland?

The Irish government was explicitly opposed to Brexit for all of these reasons, but failing that, the least it could do was to insist on a backstop which prevented the re-emergence of a hard military and customs border in Ireland in all circumstances. Unionist commentators are now arguing that Ireland has overplayed its hand and now faces the prospect of a hard border re-emerging on the 29th. of March should a no deal Brexit occur. This would be economically very damaging for Ireland, North and South, but even these economic considerations are trumped, for the Irish Government, by the political implications of the re-emergence of a hard border for peace and stability on the island.

Nationalist commentators on the other hand, point out that the crushing defeat of May's deal has made a reversal of Brexit much more likely, and that this was, rightly, the Irish governments preferred outcome all along. Either way, it is a high risk, high stakes, poker game, with the outcome still in the balance. It is a game which the Irish government very much would have preferred not to have to play in in the first place, but the historical and political situation gave it no choice but to go all in and play every card it had.

There will undoubtedly be huge recriminations within the Irish political system if a no deal Brexit occurs and huge damage is done to the Irish economy. The EU will want to secure its external frontier with the UK within Ireland, and yet it will be politically impossible for the Irish government to comply. It has even been suggested that the Irish government would pay EU fines rather than seek to enforce controls at the border. Parallels have been drawn with trade across the East/West German border prior to formal re-unification and incorporation into the EU.

At the very least, stringent controls will have to be enforced at Irish air and sea ports to prevent UK goods entering the rest of the EU via the "back door" of N. Ireland. This would make legitimate Irish exports to the EU subject to "rules of origin" checks to ensure they hadn't originated in the UK and thus could cause some delays and costs. However there are very few UK exports to the EU which could be shipped economically via N. Ireland and the Republic and so spot checks on specific products, and trucks known to have originated outside of the Republic would probably suffice.

This would not prevent UK goods having access to the Irish market via the border. However the vast majority of these are shipped directly from Britain through Irish air and sea ports, and even those which do cross the border are largely imported by major supermarket chains like Tesco which could be subjected to onsite inspection and VAT like duty payment systems. The amount of goods, subject to WTO tariffs, transported across the border by private individuals and small businesses is trivial in an EU context and will probably be officially ignored for quite some time.

The Irish government's case to the EU will probably be that the "No deal" scenario is temporary, subject to ongoing discussion between the EU and UK, and there is no point in putting in expensive and complex permanent solutions to what might well be a transient problem. Many - and soon a majority - of citizens in the North are eligible for Irish, and therefore EU citizenship and should not have their freedom of movement hindered.

So the argument that Ireland and the EU's hard line insistence on a backstop has backfired is dependent on a number of misconceptions:

1. Firstly the backstop was never primarily about trade in the first place. It was about peace and stability in Ireland, and therefore essentially non-negotiable.

2. Secondly, to have placed a time limit on the backstop would have exposed the EU to a lot of pressure to give the UK unfettered access to EU markets - in order to keep the border open - after the transition period when the EU would normally only give such access  to a third country at considerable cost.

3. Thirdly, while the back-stop was the issue which provoked DUP opposition to May's deal, only 45 Tory MPs gave it as their primary reason for voting against the deal. So even adding a time limit would not have come even close to bringing the deal across the line.

4. Fourthly, while the crushing defeat of May's deal has made a no deal Brexit more likely, it has also made the Irish government's preferred option - reversal of Brexit - more likely.

5. Fifthly, even in a worst case scenario, where a no deal Brexit occurs, post Brexit negotiations will undoubtedly take place, and the Irish Government's and EU's red line of an open Irish border will have been burnt into the political landscape. The UK will not get a deal negotiated post Brexit that will create a hard border any more than it could before.

6. Lastly, the DUP will not hold the balance of power forever. A future Labour led government, or even a Tory government not dependent on DUP support will not be slow to jettison DUP interests if the UK's economic interests require it. Ultimately, very few in Great Britain care a jot whether N. Ireland remains in the Single Market and Customs Union or not, and customs controls in the Irish sea will barely raise an eyebrow.