Relegation will cost Saints around Â£2million and Grayson admitted jobs will be lost in order to balance the books.
He said: "This outcome been a topic of conversation for a long, long time.
"When you have to plan out a season and take account of relegation then you've already gone through part of the grieving process.
"What disappoints me is we got ourselves in this position in the first place - we should never have been here.
"It's been too late since mid-February. We've struggled to win games and there were losses at home to Quins and Worcester.
"But ultimately you get what you deserve and we deserved what we got.
"What hurts me most is some people will lose their jobs and that's hard to take. But the world keeps turning.
"The players haven't produced and we take responsibility for that collectively. As someone sitting at the top of pile I also have to take it on the chin.
"The players are desperately hurt in the changing room and if they could start next season now they would.
"Injuries, poor form and some documented stuff behind the scenes put us in the position where relegation was a possibility."
Northampton chairman Keith Barwell will begin a major review into the club's nightmare season on Monday and apologised in his programme notes.
All of Saints' top players have indicated they will stay at Franklin's Gardens next season but Grayson was uncertain over his own future with a new coaching structure under consideration.
He said: "Until someone tells me otherwise I'll be here. I'd be happy to work underneath someone else.
"I haven't got an ego, I just want to be good at my job.
"I'm an inexperienced coach and I've made mistakes this year but there are some things the club needs to get right to make sure this doesn't happen again.
"The club needs to live up to the support it gets. Our supporters are unbelievably loyal. They're cross and rightly so but we'll come back next year."
London Irish director of rugby Brian Smith commiserated with Northampton and claimed their exit from the Premiership underlined the strength of the league.
He said: "We were looking for bit of closure on our own season.
"We started up a huddle on the pitch after the match but realised it wasn't the place to do it so we headed off into the changing room.
"We are disappointed we did not win the game but that is insignificant when you understand what happened to Northampton today. Saints are a good side.
"They've obviously had their problems this season but the two teams in the dropzone have won well at home today which speaks volumes for the standard of the league."
Smith admitted the Exiles' own season was tinged with disappointment.
He said: "While expectations were very high and we missed out on a top four finish, our first target was to finish in Europe, whatever shape the Heineken Cup may take.
"We did that with a game in hand. It was a good season but obviously you want to be in the play-offs but we're not, so our disappointment is based around that."