Ireland earned a famous win over the West Indies in their first of two Twenty20 internationals at Sabina Park.
From eight for two, Ireland powered to 117 for four to win with five balls to spare as Ed Joyce top-scored with 40 not out.
Windies captain Darren Sammy won the toss but his side struggled to build momentum, save for two early sixes from Dwayne Smith (14).
He and Gayle put on 31 before falling in successive overs and wickets continued to fall at regluar intervals.
Cusack was the pick of the bowlers with two for 17 in four overs, figures matched by O'Brien in two while Murtagh took two for 28.
But Joyce put on a steady 29 with Gary Wilson before the crucial partnership, 58 with Andrew Poynter.
Poynter hit six fours in an innings of 32 from 30 balls before being bowled by Sunil Narine.
Ireland still needed 22 at that stage but Joyce, whose watchful 49-ball innings contained just two boundaries, was joined by O'Brien (15no) to see them home.
The teams meet again over 20 overs on Friday before a one-off one-day international on Sunday, also at the Jamaican venue.