I hope your guys can speak German.
The mistake your time travelers made was not accounting for ALL of the assault rifles and their ammo. Not long after they left, a small group went looking for vengeance and succeeded in assassinating Cromwell using the assault rifles.
With the strength and prestige of the protestants in shambles after the defeat in Ireland and losing their leader, Charles II returned earlier than in our own timeline, established freedom of religion and openly became a Catholic in order to further an alliance with Louis of France against the Dutch and Spanish. The Glorious Revolution in 1688 never happened and in the next century after some cross-marriage, the result was one united Roman Catholic Anglo-French kingdom. Ireland lived in relative peace during this time.
The protestants finally saw their chance when the French revolution broke out in Paris and attempted to spread the revolution to London. They were however defeated when the french Royals decided to abandon France and move their army to London, aided by an Irish army that was not keen on a Protestant revival.
Scotland did successfully declare and keep its independence, aided by the Dutch Republic. The Americas did not even face any serious opposition when they declared independence as England was too focused on its internal struggles.
Skip forward to the 20th century and the Monarch of the Anglo-French Kingdom has been focused on staving off a revolution for a long time now. It stays neutral in the first World War and when the second breaks out, it simply does not have the industrial base to fend off the German invasion. The conquest of Ireland is inevitable at that point, with the German air force flying uncontested over Dublin and Belfast.
Without a bridgehead in Europe, the United States sees no path to victory and while they are officially at war with Germany after the declaration of war on its ally Japan, in practice the two countries leave each other alone until the United States defeats Japan and they sign a peace agreement.
The world is now shared between 3 superpowers: Germany in Europe and Africa, the United States in the Americas and the Pacific and the Soviet Únion in Asia.
There is a sizable German presence on the Atlantic coast, but otherwise Ireland is again left in peace and mostly ignored. War will break out soon over the Germans' insistence on replacing Guinness with "proper beer".