I think the issue here is that you're looking at a single factor to explain something that has multiple interdependent contributing factors.
Some of the factors that contribute to how easy it is to unify/conquer an area - in each case, assume that everything else is more or less equal:
- Technology - the group/tribe/nation with more advanced technology tends to have an advantage.
- Unified/Not Unified - the group/tribe/nation that is more unified at the time of battle/war tends to have an advantage
- Numbers - the group/tribe/nation with more people capable of fighting tends to have an advantage
- Terrain - areas with more natural defenses tend to be harder to conquer than areas that are more open.
As the answers to this question point out, European geography has a lot of natural defenses compared to many other parts of the world. It also has a climate that's both relatively mild and relatively forgiving - drought is rare, soils have a lot of nutrients, and the early crops were able to be grown there. That allowed many peoples to settle there, as well as allowing relatively easy trade and relatively easy defense.
When the Rus principalities were conquered by the Mongolian Horde, the Horde was able to field a much larger fighting force, was fighting largely in the terrain they were most familiar with (the Russian Steppes don't really end until near Hungary). In addition, the Rus principalities had few natural defenses.
Similarly, Ireland is relatively flat and open compared to the rest of Europe, so once the English/British forces had enough of a presence, they were able to conquer the less unified, less numerous Irish (It's worth noting that England was unable to conquer the much more geographically challenging Scottish terrain - Scotland did not join Britain until the death of Elizabeth I, when the King of Scotland became the King of England through inheritance. )
In India, the British had superior technology to counter the more numerous peoples of the Indian states, and again, relatively few natural defenses once they had a large enough presence on the subcontinent.
By comparison, in most of Europe technology was more or less at the same level, as was population. External invaders were able to penetrate deeply into European heartland: the Mongolian Hordes in Hungary; the Ottoman Empire in the Balkans, Hungary and to the gates of Vienna; the Roman Empire setting its effective northern border on the Danube.
My theory is that in mainland Europe, the various tribes were close enough to equal in terms of technology, manpower, and cultural adaptability that European geography provided enough of a barrier to prevent any one group from gaining lasting superiority.