McCarthy's current Connacht contract expires next summer.
He started Ireland's Tests against South Africa and Argentina last month, featuring in the second-row.
"We are bitterly disappointed to lose Mike McCarthy, who has been a popular senior member of our squad," Connacht chief executive Tom Sears said.
"While disappointed, particularly given the continued success of Connacht in recent years, we respect the individual player's decision and wish Mike every success in his future playing career."
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