Why not just get rid of England? Keep Wessex, East Anglia, Northumbria, Mercia, Cornwall, Danish vassals, etc. Then just have Ireland unite into a single country and then possibly conquer some parts of Britain and you're set.
Your challenge is to have Ireland become the dominant of the two major British Isles. Simple as that. Have fun.
Bonus points for independent Scotland and Wales.
Two possibilities:
1. The Romans decide the British Isles are of vital importance to the Western Empire*, and with sustained effort conquer the entire British Isles (OTL they made it right up to the Highlands) including Hibernia, the Highlands, Islands etc. But they soon find it impossible to maintain control, with the local tribes essentially waging proto guerilla warfare on their garrisons. For the first time in their histories, ancient enemies unite to repel the invaders, gaels and picts and brythons unite, forming an unstoppable tide, the likes of which was unseen since the days of Boudica. After months of bloody warfare they drive the romans right back to the coast, where they flee towards France. However the tribes know it will not be long before the Romans invade once again. They take it upon themselves to follow the Romans across the channel, forming what is a rather impressive fleet of ships for that time period, however at the time they are not concerned with naval warfare, merely with driving the Romans back and giving them a battle they will not forget! As they reach Breton they are joined by Gauls, and establish something akin to the OTL Gallic Empire. The hero of these battles is an Irish tribal chief, who won the first victory against the Romans. As the army amasses under his banner, stories of his courage and strength are told and retold, and he is worshipped almost as a God, remembered contemporarily as an Arthurian figure. For the next decades the tribes unite together to repel Roman attacks, at times even conquering their way far into Gaul. As the Roman Empire dimishes the tribes resume some of their old quarrells, and undergo a few rebellions and local feuds. But the emergent threat from Vikings, Normans, Angles, Saxons and Franks keep the Isles in an uneasy state of solidarity. (And crucially the Britons don't invite Saxons mercenaries over, as per OTL). Although it is still the southeast of England that builds up as the main port and trading post with southern europe, other trading posts and major settlements arise in Scotland and Ireland, with the chiefs (or even High Kings?) bringing back wealth from their forages into Europe.
2. With the same scenario of Roman occupation of the British Isles, Magnus Maximus is Irish and not Welsh. He never invades Italy as he suffers from sunburnbut instead holds on to his territories of the British isles and Gaul. Although later he loses gaul after a succession of Norman invasions, the kingdom of Albion remains strong until later centuries.
A few hundred years later..
Following the wave of revolutions across europe, groups of peasants from OTL England begin to revolt, angered at what they perceive to be inequalities between the native Brythonic and Goidelic speakers, leading from political agitation to full scale riots. Various revolutionary leaders emerge, from Wat tyler through to Oliver Cromwell (or whatever the Brythonic equivalent!) who conquers much of the mainland and establishes the Commonwealth of Britannia. As the civil war devastates the population, a truce is agreed. The land is divided between Britannia (OTL England) and Albion (Scotland, Ireland, Isle of Man, Wales). Scotland has been settled by the Gaels from the Highlands throughout the centuries, and Wales (known to the Britons as Cymru, and to the Gaels as An Bhreatain Bheag is a disputed territory conquered and settled during the civil war, which is later granted devolution and then independence.
*Various possible reasons, the Romans wish to expand into Scandinavia and see Ireland and the Highlands, Hebrides, Shetland, Orkney etc as ideal outposts. Or they mistakenly believe the British Isles are rich in gold or diamonds. Or (possibly a bit ASB) they have discovered North America and set up ports and settlements on Irelands West coast to trade, or dispatch military. In this case Ireland becomes densely populated, wealthy and powerful.
I don't think the scenario that involves Roman conquest of Britain would ever happen. The Romans had neither the capability nor the will to expand all the way through the British Isles. Maybe through Scotland, but no chance of reaching Ireland while there are still external threats that are major enough to warrant Roman attention towards them. There are almost no times they'd even consider such a thing.
Suppose William the Bastard is lost at sea in 1066, and lands with his army on Ireland instead, conquering and fortifying the isle step by step, forging it into a strong united kingdom.
The distance to Normandy makes the personal union end at the death of the Conqueror (as historically with England, but no third son will reunite the countries under one ruler here).
This Norman-Irish kingdom will, during the following centuries, exert a strong influence in Scotland and Wales, to make sure that one pro-irish ruler holds each country, and that no English advances on these areas will succeed.
Perhaps the Celtic-speaking areas of Cornwall and Cumbria could be ripped off the English jaws as well.
Sorry but definitely not to Scotland would the Romans go. In OTL they could barely hold on to the most northern edges of Britannia, let alone to full Scotii/Pict forces that would raid the Roman camps. Rome would have to settle for a miniature "Plot" of land in Britannia and fortify that enough to stave off those raids, but in this ATL they never got the chance.
Rome could have done it pretty easily *IF* there were no exterior threats. There are always exterior threats, and so no Empire/State will EVER be able to throw all their resources into a war.
I don't think the scenario that involves Roman conquest of Britain would ever happen. The Romans had neither the capability nor the will to expand all the way through the British Isles. Maybe through Scotland, but no chance of reaching Ireland while there are still external threats that are major enough to warrant Roman attention towards them. There are almost no times they'd even consider such a thing.
Sorry but definitely not to Scotland would the Romans go. In OTL they could barely hold on to the most northern edges of Britannia, let alone to full Scotii/Pict forces that would raid the Roman camps. Rome would have to settle for a miniature "Plot" of land in Britannia and fortify that enough to stave off those raids, but in this ATL they never got the chance.
Rome could have done it pretty easily *IF* there were no exterior threats. There are always exterior threats, and so no Empire/State will EVER be able to throw all their resources into a war.
Suppose William the Bastard is lost at sea in 1066, and lands with his army on Ireland instead, conquering and fortifying the isle step by step, forging it into a strong united kingdom.
The distance to Normandy makes the personal union end at the death of the Conqueror (as historically with England, but no third son will reunite the countries under one ruler here).
This Norman-Irish kingdom will, during the following centuries, exert a strong influence in Scotland and Wales, to make sure that one pro-irish ruler holds each country, and that no English advances on these areas will succeed.
Perhaps the Celtic-speaking areas of Cornwall and Cumbria could be ripped off the English jaws as well.
To get that lost at sea between Saint-Valery-sur-Somme and Pevensy would require ASB intervention. Simply put, if he ends up off course, the simple shape of the channel means that he will either make landfall in Belgium, the Netherlands or some other part of Eastern England if he's blown East, or Normandy, Brittany, Cornwall or some other section of the south coast if blown west. To get to Ireland would require a deliberate and major change of course that William would be unlikely to go for and may not have had the supplies to do.
This can't really be done. The greater Thames valley area is always going to be the richest part of the British Isles due to be the landscape encouraging a single state, the waterway being the closest to mainland Europe for trade, and the land being so much more fertile than any other part of the British Isles.
Let me put this in terms our American readers will understand. In population terms, this would be like Canada taking over the United States (in 2012). At the peak of Ireland's population catching up to Great Britain's, in the 1840s before the potato famine, it would be like Mexico taking over the United States (in 2012). The latter is a better comparison in cultural terms.