When Jill O'Connell heard her sister needed a kidney transplant, she didn't think twice about becoming a donor. Soon the siblings became the first ever identical twins to undergo the procedure at Dublin's Beaumont Hospital.

Two years on, the sisters are fighting fit, and Valerie O'Connell will never need another transplant, or have to take anti-rejection medication again.

Speaking to the Sunday Tribune ahead of the Irish Kidney Association's 25th Annual Service of Remembrance and Thanksgiving for donors yesterday, Jill was in humorous mood.

"Obviously no one else was tested, because when you are twins you are born with spare parts," she said.

"I said, 'Make sure you put an indelible marker on mine in case I need it back.'"

The twins' awareness of the necessity for donors was raised 20 years ago when doctors discovered a problem with Valerie.

"It was only when the two of us went to start up a pension fund that the medical found that Valerie had protein in the urine," she said.

For nearly two decades Valerie visited doctors for check-ups. Then, at the age of 40, she was finally told she needed dialysis and found herself hooked up to a machine three days a week.

"It got to the point that they decided we would do a kidney transplant," said Jill.

"She was always getting my kidney, it was never really discussed. She needed it and she was going to get it."

What followed was something the dedicated transplant team at Beaumont had never before dealt with. The operation was a complete success, straightforward and without post-surgical complications.

"The last three years before the transplant took place, I looked 38, Valerie looked 58. Her skin was grey, and then within 48 hours everything changed," said Jill.

"I was given Panadol and she was given Panadol and that was the height of our medication after the transplant.

"It was a Wednesday and I came out of the hospital on Saturday and Valerie came out on the Sunday. That was unheard of. It's amazing to see how she is now. She has her life back."

Yesterday's remembrance service at Corpus Christi Church in Drumcondra, Dublin – attended by President Mary McAleese, Archbishop Diarmuid Martin and Dublin lord mayor Gerry Breen – was literally about mixed blessings.

On the one hand there were the grateful families who received rare transplant organs. On the other, the families who, in the midst of devastating loss, made sure others benefited from the deaths of their loved ones.

Jill O'Connell said she would be a donor all over again, if it was possible, but was amazed to learn other people in her situation do not see the decision to donate as so straightforward. "I met a guy over the summer who needed a transplant and his sister was a match for him but she wouldn't give him the kidney. Beaumont said that wasn't unusual even though I couldn't believe it myself," she said.

"It's a major operation, but it doesn't take that much out of you. If I had another kidney I would do it again for a friend.

"These people's lives are run by machines. You don't need two kidneys. Only God knows why we have them."