McCarthy is set to be promoted from the club's academy next season, alongside Rory Scannell and David Johnston.
Foley said: "This news speaks volumes about this group of players. They are committed to the province and are ambitious to achieve something together.
"Although still young, Duncan is an experienced player and we look forward to seeing him make a full return from injury, while with Sean it's great to see another academy player make the step up.
"We also look forward to welcoming Sam to the squad as he has shown to be an exciting prospect for the future."
Munster are sixth in the GUINNESS PRO12 table, though are just four points off the summit and return to action next Sunday with Ospreys the visitors to Irish Independent Park.
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