In regards to Wales it has to do with their succession laws. Hywll dda creates welsh law. He united Wales under one banner, but decided that land should be shared unequally among his sons. One son would gain the heart and biggest parts of Wales and any other sons would get the divided rest. This lead to constant battle between Welsh kingdoms to establish themselves as the main power in Wales. While this did create some of the strongest men of the time, like Llewelyn the great. Who unified Wales and made agreements with King John station that Wales is fully under Llewelyn’s control not Jons. It also meant that when men like Llewelyn weren’t around, no one man strong enough to unite the kingdoms. The kingdoms just kept fighting each other until someone strong enough did come along. There’s a saying, the only thing a Welshman enjoys more than killing an Englishman is killing another Welshmen. The petty kingdoms of Wales were ravaged by its succession laws and meant they were never fully unified to defend against the eventual Norman invasion that conquered Wales. This is in regards to Wales only. Sorry for any errors in grammar, wrote this on my phone.

I don't know much about Wales, and not enough about England or Scotland to fully contrast their development with that of Ireland, but I'll try to give you a picture of Ireland's movement towards political unification and offer some explanations as to why it wasn't complete by the time of the Anglo-Norman conquest in 1169/70.

It's a well established aspect of pre-Norman Irish history that the power of kings and over-kings gradually increased throughout the early medieval period, with the countless petty kingships (tuatha) that dotted the country coming under the control of a small number of more powerful 'provincial' kingships by about the 7th century. By the mid-8th century the Ui Neill of Tara and the Eoganacht of Cashel dominated the northern and southern halves of the island respectively, and by the mid-9th century the Ui Neill kings could call themselves 'king of Ireland' and assert their will across large swathes of the country, with their power only occasionally challenged by defiant Eoganacht upstarts and Viking warlords. Note, however, that I say kingships rather than kingdoms - these were hegemons over a hierarchy of autonomous polities, not rulers of centralised states, and these petty tuatha remained the basic political units of the Irish landscape throughout this early period. Moreover, there was no single Ui Neill or Eoganacht 'dynasty' ruling the north or south of the country, but rather a number of competing Ui Neill/Eoganacht branches fighting for supremacy. Among the Ui Neill, for instance, the hegemonic kingship of Tara alternated between the rival northern and southern branches (which were themselves subdivided into even more rival sub-septs), preventing their consolidation into a unified power.

This political order began to transform in the 10th century as Viking settlements along the coasts developed into permenant towns with strong trade-based economies and powerful mercenary armies. These towns fell under the control of local Irish rulers like the Dal Cais, granting military, naval and economic strength to their rulers and causing a massive shift in power that enabled Brian Boru, drawing on the resources of Norse Limerick, to overthrow the hegemony of the Ui Neill and bring the whole country under his control in the early 11th century. He took hostages from all the major Irish kingships, a first in Irish history, and was even declared 'Emperor of the Irish'. Yet his power remained personal, and after his death in 1014, and the death of his rival/partner Mael Sechnaill II in 1022, any political unity in Ireland fell apart.

What emerged then in the 11th and 12th century was a group of increasingly centralised and powerful provincial kingdoms, chiefly Munster (until its division in 1118), Leinster, Connacht and Tyrone, each vying to dominate the whole island and occasionally succeeding. Compared to the provincial kingships of early centuries, which were merely regional hegemonies, these rulers were able to consolidate a strong degree of political unity within their provinces, backed up as they were by growing military and economic power. The most powerful of these kings recieved the submission, usually in the form of hostages, of their rivals and attained the status of 'high king'. Some of these so-called 'high kings' could even expand outside of Ireland, establishing control over Mann or the Hebrides (both politically linked with Dublin). They began to 'Europeanise' their kingdoms, enacting church reform, patronising Romanesque architecture, and engaging in large-scale works of military engineering like building stone fortresses or fording/diverting rivers.

Nevertheless, unity had not been achieved by 1169, when the Anglo-Normans were drawn into the rivalry between Connacht and Leinster, leading to their conquest of the country. Despite many high kings achieving real dominance over the country, none had been able to permenantly break the power of their rivals and establish a permenant hegemony. Once a high king died or fell ill there would inevitably be a free-for-all competition for power between potential succesors and rival kingdoms, preventing the consolidation of any single province/dynasty's rule over the country.

The question then is why, despite this consistant move towards greater kingly power, Ireland hadn't been unified even several centuries after England and Scotland had been. Generally Britain and Ireland experienced similar forces, internal and external, driving political consolidation - Christianisation and the growth of wealthy ecclesiatical sites, agricultural expansion, technological advancement (eg. watermills, heavy ploughs), contact with the Carolingian world, integration into the Norse economic world and the subsequent growth of trade and urbanism, etc. Ideologically, the idea that there ought to be a single king of Ireland had been around since c. 700, about the same time that Bede envisioned a single English identity. So in a lot of ways it seems like Ireland should have developed just as fast as England and Scotland.

So why not? The lack of primogeniture in Gaelic kingship is one major factor, as it led to the splitting of dynasties into many rival branches. However, given that Scotland developed out of a Gaelic(ized) kingdom, I don't think a simple cultural explanation is enough, even if it does play a very significant role. Neither do I buy the idea that geographical features like the midland bogs prevented unification, given that navigable rivers like the Shannon, Liffey and Boyne led right into the heart of the country. Nor is isolation from Europe particularly meaningful considering Ireland's close contacts with the Carolingian world and subsequent centrality in the Norse world.

In regard to England, I would say it largely comes down to the Anglo-Saxons having a significant head-start when it came to political growth. Like I said, I'm not very familiar with Anglo-Saxon history, but my understanding is that the early Anglo-Saxon kingdoms emerged within a chaotic post-Roman power vacuum, which allowed them to grow fairly quickly into a small number of large kingdoms like Northumbria, Mercia and Wessex by the late 7th century. At the same time, Ireland was still divided into some 150 petty tuatha, developing in-situ from prehistoric roots, with provincial hegemonies only beginning to emerge. Compared to the relatively large Anglo-Saxon kingdoms, these shifting hegemonies were fairly weak, decentralised, chaotic and prone to internal division. Nothing on the scale of 7th-century Northumbria existed in contemporary Ireland, nor could any 8th-century Irish ruler have built anything like Offa's Dyke. Moreover, Ireland experienced nothing like the massive Danish invasion that left Wessex as the only strong Anglo-Saxon kingdom in the 9th century, and thus in a position to unify the country. The closest Irish parallel was the collapse of Ui Neill hegemony around 1000 AD, which only gave way to a period of renewed rivalry between peer polities.

England, then, went through two periods of rapid political transformation which saw the collapse of an old order and the emergence of a new one, namely the collapse of Roman power and the Viking invasions, the first leading to the growth of large regional kingdoms and the latter to the consolidation of a unified England. Ireland went through no such sudden transformation, except perhaps with the rise of Brian Boru.

As for the contrast between Ireland and Scotland, I don't think it's actually as big as it seems. Scotland as we know it wasn't really unified in the 10th or 11th centuries, with the lords of Galloway and Moray for example being largely independent (not to mention the Kingdom of the Isles). The lands that the Scottish kings actually had secure control over weren't really that much bigger than those controlled by Irish provincial kings of the post-Brian Boru period. What was different about Scotland was the lack of strong local rivals to prevent the consolidation of power, allowing for the successful 'Normanisation' of the kingdom in the 12th century. This same sort of 'Normanisation' was basically what Diarmait Mac Murchada, the ousted Leinster king who invited the Norman invasion, had intended to do. But the politically chaotic situation in Ireland, in contrast to the relatively stable one in Scotland, allowed this Normanisation to manifest as outright conquest.

Again, I'm no expert on England or Scotland, so if I've misrepresented the development of either, somebody please correct me.