It's a case involving their European customers.
If you do business in Europe, there's a bunch of (good!) privacy regulations you have to comply with. One of these is that you're not allowed to transfer the data to a jurisdiction that doesn't follow equivalent protections to the GDPR[0]. TikTok transferred European user data to their Chinese servers, which is a pretty obvious no-go, since the Chinese government is an authoritarian watchdog that inherently can't guarantee these protections (as the GDPR also applies to transferring data to the government.)
Ireland has jurisdiction because the EU offers something called the "one stop shop" concept, where a foreign company can declare that they have EU headquarters in a specific member state, and from that point on the only EU regulations they have to directly worry about are how they're implemented in that country in specific[1]. Every major tech company is therefore in Ireland because the country is small enough to essentially steamroll local politicians with lobby money, leading to very lax enforcement until the EU starts applying pressure.[2]
[0]: This also causes issues with data transfers to the US, and in the most extreme interpretation, makes it so that you probably can't do business with both Europe and the US at the same time in the first place. This is because of the CLOUD act, which goes across jurisdictions and is something the US government can use to compel any service provider to hand over data.
[1]: Of course, a country can still have it's own laws that a company can run afoul of on top of that.
[2]: Other countries with this issue are Luxembourg (Fintech companies love Luxembourg because they can just hire all the good lawyers, meaning you can't negotiate legal disputes there effectively) and the Netherlands (which is a EU-based tax haven for large corporations that aren't in either sector.)