The 1656 Treaty of Dublin was the document that originally founded the UK and formally ended the HRE, which was devolving into chaos after nearly 50 years of non-stop civil war. In exchange for Austrian and Anglish permission to leave the HRE peacefully, as well as Anglish help against Scandinavia, Brandenburg ceded the Netherlands (the province of Loither) to the Anglish, while Austria received Brandenburgish help in the war on Bavaria. One provision that both Brandenburg and Angland insisted on was the incorporation of Eire into the UK. Eire at that point was also in civil war, with the King recently killed, and the Regent nearly powerless. Despite being firmly against the deal, as they felt this would throw the balance of power off, Austria was at the time locked in a massive war with Bavaria, as well as a looming one with Spain, and urgently needed Brandenburgish help with these. Brandenburg, meanwhile, was firm on this point, as they needed to destabilize their Scandinavian enemies, who were close allies with Eire. At the same time, Angland would then enter the war, allowing them to win.
Following the disastrous war with Brandenburg in 1583-6, Angland's empire collapsed, being reduced just to the Island of Britain. Since then, Normandy and France had been reclaimed, down to Poitiers, as had Iceland, but Angland was now in the middle of a political crisis, with the army barely able to hold what they had, as well as public outcry against incompetent rulers. Many urged that the province of the Netherlands (including all of the Low Countries) should be attacked and annexed, as this would also bring in large amounts of gold, but King William III remained wary of engaging in another war against Brandenburg. Still, without some new gains, it was feared that the great Anglish Empire would never rise again.
Austria was also in a desperate crisis. Having spent the last 100 years fighting enemies within and without the HRE, its finances were in ruins and its army exhausted. Meanwhile, it was fighting a losing war against Bavaria and its Schismist allies, while the north had started to secede. Saxony had seceded almost 15 years ago, but Austria had been unable to stop it due to Brandenburgish interference; effectively, had Austria attempted to attack Saxony all of North Germany would secede and rip the HRE apart. With the war in Bavaria, it was acknowledged that it would be impossible to fight on both fronts at once. Since then, though, most of the north had seceded, and now Brandenburg was attempting to, only stopped by the fear of Angland intervening with Austria, its old ally. Nevertheless, with the war in Bavaria having now reached new lows, and the Spanish split from Austria (which was almost certain to lead to war), Austria needed a way out from this struggle.
Brandenburg, despite seeming to have the upper hand, was in fact in worse shape than either of the others. The HRE civil war, the Polish wars, and now the Scandinavian War had chipped away at Brandenburg's world standing and finances. The Russo-Norse union fighting Brandenburg was almost certain to defeat them, possibly even to the point of annexation, and with parts of its provinces in open rebellion - including the Netherlands - it was on the point of collapse. it needed aid against Scandinavia and Russia, a way to leave the HRE without sparking yet another war, and a way to quell the rebellions in its provinces.
The Netherlands, meanwhile, in open rebellion, offered a ceasefire between its troops and Brandenburgish ones, and the temporary control of the Netherlands to Brandenburg, if its leaders were allowed to the conference.
Eire was united into a single union in 1486, but after a brief period of peace, the country plunged into civil war in 1542 and hadn't emerged out of it yet. When the last King died, in 1611, a Regency was put in place. Over the last 50 years, though, all it succeeded in doing was losing all the land in Eire except that between Dublin and Belfast. Now, one of the loyal Earls in Central Eire had managed to get a stable army up, and suggested that, with some help, Eire could be united there, provided that some outside nation came in to help. The only one present being an expansionist Angland, well ...
The first round of talks between the five parties was actually held in London, not Dublin, due to fears that the enemy armies would overwhelm the precarious defenses around the city. By the second round, English troops had cleared the armies around Dublin, and the talks were shifted there.
During the initial talks, arguments between the Netherlands and Brandenburg destabilized the talks, with Austria siding with Brandenburg and Angland and Eire siding with the Netherlands. In the end, the Anglish delegation managed to get a concession out of Brandenburg that the Netherlands would be given independence; in return, Anglish troops were immediately deployed to Danemark, attacking the former Anglish province. The resulting shift of Scandinavian troops managed to divert enough troops for Brandenburg to press on Russia. By the end of the talks, St Petersburg had been seized. However, this was not enough to deflate tensions, and with the problem of the post-independence Netherlands unsolved, the conference was ended.
The second round of talks, held in Dublin, were the most useful of them all. At the very beginning of the talks, Brandenburg agreed to send troops down to Bavaria to aid in that war, alongside Austria, in return for which Austria and Angland agreed to break up the HRE and allow Brandenburg to leave. Moving on to the issue of Eire, Angland hinged their participation in Brandenburg's war with Ruscia and Scandinavia on the Anglish annexation of Eire. Brandenburg agreed to this, as, tacitly, did Eire, but Austria, worried about the balance of power in Europe, insisted that if such a measure was taken, then the HRE could not be dissolved. Finally, after a month of argument on the topic, Angland threatened to walk out of the talks, along with Eire. This would have disastrous consequences for all others - without Anglish support, the Netherlands could not survive, and without Anglish help, Brandenburg could not win the war against Ruscia. Austria would not be in a worse position than they currently were, but then their current position was untenable as it was - they were swiftly losing a war that they should have won easily. With great reluctance, Austria agreed to the deal. With that, the conference came to an end.
The Final round of talks had initially been meant just to formalize the treaty, but by the time they came around, no agreement had been reached about the Netherlands. Eire had been formally annexed to Angland by now, and Austria signed the treaty immediately, seeing as the Netherlands was none of its concern, leaving just Angland, the Netherlands, and Brandenburg. In the first round, Brandenburg had agreed to release the Netherlands, but had not done so yet, instead choosing to maintain the ceasefire for the year since then. Now, the Netherlands announced that it wished to immediately leave Brandenburgish territory and be acknowledged as a sovereign state, but Brandenburg refused, worried that if the Netherlands was freed, it might join the war against Brandenburg. Angland quickly mediated, however, and it was agreed that the Netherlands be given independence, and in return, the British would give a guarantee against them. The Treaty of Dublin was then finally signed.
Despite the help of Brandenburgish troops in the Bavarian war, Austria was unable to defeat the Bavarian armies. In 1672, finally, Bavarian troops marched into Vienna and executed the King. Brandenburgish troops had been pulled out of the war in 1670, after a grievous loss against the Bavarians resulted in the deaths of almost 50,000 Brandenburgish troops. Following the defeat, the Bavarians placed a puppet King on the throne, though due to Brandenburgish action, the puppet King only controlled southern Austria, while Brandenburg set up a rival kingdom to the North, with the capital in Prague.
Thanks to Anglish help, Brandenburg crushed the Ruscians and the Norse in four separate wars, the last one in 1714. It is generally considered that it was these wars that led to the Muscovite rebellion, and later the the Second Ruscian Civil War. Meanwhile, with the Netherlands freed from Brandenburgish control, it was easier to neutralize the other rebellions, and Brandenburg did so with ruthless efficiency. Following the destruction of Austria in the Bavarian war, Brandenburg and Bavaria ended up in a Cold War, only ended when, in WWI, the Treaty of Berlin unified them into the German Federation.
The treaty revitalized Angland, and soon after, the old Anglish Empire was effectively reconstituted, once Danemark was won back from the Scandinavians. The United Kingdom formed become a foremost world superpower, a title which it still holds today.