This offer is not available to existing customers of Airtricity, they only offer 8% off the Gas and Electricity to existing customers and no other offers are available to them.
What you need to be doing is comparing the actual rates and standing charges, not necessarily the "percentages off" that companies advertise.
I currently have my gas with Flogas who are the cheapest option at the moment. No need to sign a contract with them either.
Best for Electricity at the moment appears to be Bord Gáis or Airtricity. However, the Airtricity offer is for new customers I think and involves a 2 year contract.
I would avoid signing a 2 year contract with anyone if I were you. You want to be able to review the prices in 9 months-1 year and maybe move again.