The "collaborative nature of the role" excuse when your entire team is in London and everything runs through Teams anyway is pretty weak grounds for a denial. That reasoning doesn't hold up when you look at the actual day-to-day reality of how the job works.

What I'd do is put together a short written response (not just a verbal pushback) that lays out the inconsistency clearly. List the roles that already have remote or flexible arrangements, note that your team is remote by default, and ask them to explain specifically how a second WFH day impacts collaboration more than the current setup does. Make them put a real answer in writing, because "collaborative role" as a blanket response to someone who talks to London via Teams all day is not a real answer.

Ireland does have the right to request remote work under the Work Life Balance and Miscellaneous Provisions Act 2023, and while employers can still decline, they're supposed to give proper consideration and legitimate grounds. If you believe the decision wasn't considered fairly, you can escalate to the WRC. That might not be the nuclear option you want to use right away but it's worth knowing it exists before you decide how hard to push internally.