Irina Shayk -- My Cups Runneth Over In Cancun

Here's international supermodel Irina Shayk ... showing off her INSANE physique during a beach trip in Cancun, Mexico.

The 29-year-old's bikini top battled heroically to stay in position as Irina splashed around in the surf ... and while there were several close calls ... it ultimately emerged victorious.

Of course, Shayk recently broke things off with her longtime soccer superstar boyfriend Cristiano Ronaldo -- who's gotta be kicking himself right now ... or banging someone else.