As was mentioned by papabravowhiskey, if you bought two separate tickets and cleared immigration and customs in ROI then checked in for your next flight to the UK, you have officially entered the ROI and have for all intents and purposes ended one journey and began a new journey from the ROI to the UK.
I have travelled numerous times in the past from the Middle East to the UK via AMS/FRA/CDG but on separate tickets, so I had to clear immigration and customs on the continent and then check in again for flying to the UK. My new baggage tag now had a green border showing this journey originated in the EU and allowed me to use the blue customs channel for intra-EU travel on arrival in the UK, which is no different to someone who began their journey in AMS/FRA/CDG.
Using the same analogy here, it would be permissible for you to claim you started your journey in the ROI, given the situation I described.