PORTRUSH, Northern Ireland — Even in the pin-rattling wind and the heavy rain that often seemed to be falling horizontally, Shane Lowry made winning his first major championship look as natural and easy as his light-up-the-clubhouse grin.

He had a four-stroke lead heading into Sunday’s final round. He increased it to win the British Open by six strokes, finishing at 15 under par and transforming the typically fraught closing holes into a merry march through the drizzle as his compatriots — Irish and Northern Irish alike — accompanied him with song, roars and good cheer.

Toasts — surely many toasts — will be forthcoming.

“Portrush could be pretty messy later,” said Graeme McDowell, a Portrush native who won a major of his own at the 2010 United States Open.

But don’t be fooled. Golf is “fickle,” as Lowry, a 32-year-old with a thicket of a beard, reminded his audience as the claret jug awarded to the champion shone brightly in front of him.