Mr Bobby Molloy said yesterday he resigned because it was the right thing to do and he didn't want to damage the Progressive Democrats party.
He said it was only when he got away from the "media frenzy" on Tuesday night he realised the seriousness of the issue. "All my instincts told me . . . this mistake is serious," he said on Galway Bay FM.
Mr Molloy said the Mr Justice O'Sullivan's comments came "out of the blue".
The news had filtered from the court to the party press officer who contacted him.
"Initially I thought, 'Oh, that was only a query about whether a letter was delivered or not'. I must say the enormity of it dawned on me afterwards, that this approach shouldn't have been made and therefore you've crossed the line. And there was nothing to do other than to apologise for it and regret it."
While he insisted on Tuesday he wouldn't resign "my own reflection on the matter was that it was an issue that was going to dominate and would cause concern to other people in my party.
"And we are facing into knocking on doors and it's obviously going to be something that's very hard to explain."
He said it would have been very unfair if he had decided to proceed and say no wrong had been done.
But, he said, he felt the perception of the event was "much more serious" than the reality.
"I didn't want it to damage the party or my supporters or to create difficulties and I felt it was important that I would do the right thing.
"The instinct to do the right thing in this case was to resign my position as Minister of State," Mr Molloy said.
PD party leader Ms Mary Harney talked to Mr Molloy and advised him to think about it overnight.
"I woke rather early in the morning and the feeling was very strong - 'You've got to do the honourable thing and do the right thing here'."
He said he appreciated the strong support from Ms Harney and the Taoiseach. "It's one that I had to figure out for myself. And I think I'm doing the right thing. I hope I am."
Mr Molloy said he was sorry if he was letting down the people of Galway.
"They've been very good to me over the years, supported me, re-elected me - I think 11 times - and certainly it wasn't the way I planned to go out of politics but I think in the circumstances it's important that I do the right thing."
