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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
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The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
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