A bit of background: The Tir TRC are already heavy hitters for a police force, regularly sporting automatic and heavy weapons. However, Tir na nOg has real heavy-hitting, anti-terrorist guys, probably the scariest in the world, developed over years and years of turmoil in Ulster. They're an elite unit of the TRC. The definition of ice cold, unlike other peace-keeping forces they even carry out assassinations. Physad initiates with access to all the highest level magical enhancements, unique hardware, deltaware, and unlimited powers of search, seizure, and arrest. This is exactly the kind of thing they might be investigating.
Also, in Tir na nOg mere membership in a group on the prohibited organization list (read: anyone labeled a terrorist) is a crime with a minimum of 2 years maximum security jail time, with no upper limit. They also have pretty strict laws on weapons and such compared to UCAS.
Final bit of background: the Path of the Wheel magical tradition is a big part of Tir society. There's all manner of symbolism for each path (cardinal directions, colors, seasons, holidays, icons) that have meaning, if you choose to work it in. A week long festival could easily be tied to these, I imagine, or the reason someone would target it.
Hopefully that can be some kind of help, either with fleshing out who might be investigating, or the stakes if the runners want to play hardball with the cops. Where does the adventure go? Maybe it's a race to get to the offenders before the TRC does (perhaps a contact has been implicated?). Or maybe one of the Danaan families (O'Neills, MacGuiness, etc.) has previously had secret dealings with the group and needs them taken care of so it's not made public knowledge. Perhaps its a case of mistaken identity, and the runners have to catch the real culprits before the TRC geeks them instead.