That is kind of like asking "Why can I pick up an egg but not fight off a cold?" The two capabilities are not related in any obvious way.
(warning - weak answer full of assertions and opinions - much of the argument is derived from Fukuyama).
The ability to project external power is based on infrastructure (UK ruled the waves - had extensive support structures to reinforce power anywhere, massive fleet size), technological/military superiority, and military skill & training. It is crucial to note the numbers of Irish and Scots in the UK military - if you try to use those forces to suppress dissident populations, you may lose both the ability to project external power and the ability to control internal dissent.
The ability to suppress dissent derives primarily from internal perceptions of the legitimacy of government. Internal perception of legitimacy results (loosely) from an accountable government, national mythology (see update below), and from prosperity. The globe spanning empire supports the national spirit and mythology.
Force projection is not related to the ability to suppress dissent (in a democratic government) - attempts to use the military on domestic populations is .. exceptionally unwise...
Ireland and Scotland are fascinating cases, but trying to analyze them through force projection capabilities is like use my spleen to lift weights. It just doesn't make sense.
update I was uncomfortable leaving an answer without a source - felt hypocritical. Fortunately, the excellent Revolutions Podcast from Mike Duncan delivered an episode about the revolution of 1830 - granted, isn't about England, but it is a fairly good case study cautioning against deploying troops to subdue the domestic population.
Update2: @TheHonRose pointed out that my use of "mythology" is nonstandard and
perhaps more anthropological than purely historical. This is correct; I'm trying to abstract a concept that is present in Fukuyama and in political science, but for which I don't have a purely historical framework. Every nation has a set of shared cultural, political, social, civil and military institutions. Fukuyama argues that these institutions, and more importantly the belief the population holds in these institutions, is a major source of the legitimacy of the government. Rebelling undermines these institutions. I'm using the term mythology in a specific sense as used in other social sciences to emphasize that it is the belief in these institutions, not the institutions that generates the legitimacy. Affiliation with the institutions affiliates with the national mythology; attacking or rebelling against the nation distances members from the group. One final point and I'll let this rest - I have no way of measuring or proving the concept, but in my opinion as an unrepentant Anglophile, the British mythology is more effective than the average, and therefore more supportive of legitimacy, and more effective at resisting domestic unrest. As I said at the beginning, most of this is from disciplines other than history, so I didn't want to emphasize it in the answer, but I wanted to capture answers to some questions in the comments. (Comments are now obsolete).