This question already has an answer here:
I have Stamp 4 on my IRP card in Ireland(formerly GNIB) and working full-time here. I am traveling from Dublin to Delhi via Heathrow Airport in London and would be changing terminals at the airport.
So, my question is, do I need to apply for a UK transit visa. My colleagues have gone via the same route and they said to me that a UK transit visa isn't necessary if you have an IRP card but just wanted to make sure.