The Englishman, who led Ireland to its first World Cup, quit Thursday after almost 10 eventful years in charge with a record of 46 wins, 30 draws and 17 losses.
"I have felt for some time there was a need for change," said Charlton, 60, after a meeting with the Football Association of Ireland.
"Nine years is a long time in the job. They have been brilliant years. I have enjoyed every minute ... but everything must come to an end."
Charlton's decision came earlier than expected. The former Leeds United defender, who lifted Ireland's soccer team from the lower reaches to the top flight of world soccer, said Wednesday: "I need to have a little think and make my mind up. But I'm being put under a ridiculous amount of pressure."
The end came a week after Ireland lost a European playoff game 2-0 to the Netherlands at Liverpool.
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