Arguably, the biggest part of it was the Irish factor. Yes, there are fighters from countries with way more people, but there are very few elite Irish MMA fighters, and so when you have a polarising guy like Conor, who is great at talking, it gives pretty much an entire nation a figure to rally behind. His winning and climbing the UFC rankings became massive news in Ireland because he's the first guy they had at that elite level. Even people who aren't aware of MMA in Ireland were mindful of Conor because he was their guy.
A similar comparison I'd make is to tennis. I'm from Scotland and know pretty much nothing about tennis. It's not often something people talk about. But I know the name Andy Murray, and any time he plays a big match, he's all over the news. This is because Scotland doesn't have lots of elite/notable tennis players, so when we do get one, pretty much everyone supports them and knows about them, even if it's someone who doesn't know anything about tennis.
All this helped push Conor into stardom, and the UFC saw that there was a huge Irish market they could capitalise on, and so they pushed him hard. Conor capitalised on all of this and was very smart about promoting himself and his career.