BUT he believes that having got through some tough days, he and wife Sally Anne now enjoy a stronger relationship.
In today's Celebrity Diary, Derry describes the affair as the "worst thing" you can do in a marriage.
Derry and Sally Anne own top Dublin restaurant l'Ecrivain, which they have run for 23 years. The couple are looking forward to celebrating their 25th wedding anniversary shortly, which Derry says they will "celebrate in style".
Derry says his affair is something that he has to live with.
"All marriages go through ups and downs and what I did was the worst thing you can do in a marriage, which is to have an affair... that's going to go down on my CV and it's something I have to face," he said.
Derry and Sally Anne have put the past behind them and are planning to fly to Hawaii to celebrate their silver wedding anniversary on October 7.
Celebrity Diary, Page 20