Originally Posted by corporate-wage-slave

There is no requirement to re-do security on a purely UK trip, so NCL to anywhere at T5 means no security. NCL to anywhere at T3 does mean security due to the mixing with other passengers. There is always a security check after arriving outside the UK, that includes Dublin, so you are best to go landside at T5 from DUB and then go through main security there from landside.