That's because they buy the highlights of the other games, which are relatively cheaply available because UEFA are selling them to every country in Europe (and many outside of Europe).
With the LoI, RTÉ have to do all the work themselves, which costs much more. It's simple economies of scale.
You're right on the LoI TV market though - RTÉ don't have a 100% share. Other broadcasters could step in and fill the gap. But they're not. What does that tell you?
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