Hello John
If you are staying in the UK for more than 12 months (from date of first entry) you need to exchange your Irish licence for a UK licence. You can apply on form D1 which is obtainable at any post office.
If you want or need to change your driving licence for a GB driving licence, you must complete the application form D1 that is available from the Driver and Vehicle Licensing Agency (DVLA) form ordering service and Post Office branches.
You will need to enclose original documentation confirming your identity and a passport style colour photograph. Send your completed application and the appropriate fee to DVLA, Swansea, SA99 1BT.
There is more information here:
Exchanging your foreign driving licence : Directgov - Motoring
And the fees required here:
The cost of a driving licence : Directgov - Motoring
Uk