I done this last year. Just ring your insurance company with the UK reg number and the length of time you want cover for i.e a couple of days or till the end of you current policy. If you go to the end of your current policy then you can get them to change it to the irish reg when you get it.
If you do it in time you should receive a letter from the insurance company stating that you are covered to drive the car before you go to the UK and bring the letter with you. Depending on the insurance company and length of time left on the policy, they might charge you a fee for doing this. I was with Hibernian at the time and it was free if you changed it on-line but they charged E20 for changing it over the phone.