Will I transit landside or airside through Heathrow when flying from Dublin to Singapore? Do I have to go through border control?

Hi everyone, I am a Thai national but I have permission to live in Ireland - I have an IRP (Irish Residence Permit). I'm wondering if I will have to go through immigration when flying from Ireland to Singapore.

I've read that if you transit from another country, you won't have to cross border control as long as everything is on the same ticket. But Ireland and UK are both in the CTA so I'm not sure how that will affect the transit, and I might need a visa if I have to go through immigration. Thanks!