Belfast is in Ireland. That is not a political opinion, it is geographic fact.
Prime retail property is insanely expensive in Dublin. Irish people do, on average, earn and spend more than people in the UK (the average income is about 50% higher) and, with the highest iPhone market share in Europe, they clearly like Apple products, but the likely success of a Dublin Apple store would still not be enough to justify the cost. That will not change anytime soon.
They have a lot more to gain from investing in less developed territories where a high-profile store helps to firm up their foothold as a premium brand and status symbol. The new Bangkok Apple store is a perfect example of that and, as the anchor tenant in a $1.5 billion development, I would guess that they pay no rent and may even have received a substantial contribution towards their build cost, perhaps even all of it - Apple's credibility adds that much value to overblown developments like IconSiam, allowing the developers to sell tiny 40 m2 studio condos, located in towers 70 stories high, for over $300,000, or a 60 m2 1-bed for over $500,000. Expect a lot more of this from Apple.