The Saltburn star attended the event with his girlfriend, Espresso singer Sabrina Carpenter.
Oscar nominee Keoghan appeared to take inspiration from the Victorian era as he sported an eye-catching look by Burberry, made up of an olive velvet tailored suit and waistcoat, teamed with a ruffled cream shirt with a necktie and a black top hat.
Meanwhile, US star Carpenter wore an Oscar de la Renta black velvet bodice with a voluminous blue satin skirt.
Actress Eve Hewson wore a custom creation by fellow Dubliner Simone Rocha. Hewson wore a pale pink chiffon gown, the skirt of which was sheer, with matching gloves.
Irish actor Andrew Scott also turned heads on the carpet by showing off his arms in a Versace ensemble, as he walked the red carpet with actor Jude Law and designer Donatella Versace.
Andrew Scott, from left, Donatella Versace, and Jude Law attend The Metropolitan Museum of Art’s Costume Institute benefit gala (Evan Agostini/Invision/AP)
Law played played Dickie Greenleaf in the 1999 original film version of The Talented Mr Ripley, starring opposite Matt Damon, while Scott plays Tom Ripley in the recently-released Netflix series, opposite Johnny Flynn as Dickie.
Belfast actor Jamie Dornan made his Met Gala debut in a morning suit by Loewe, designed by fellow Northern Irishman Jonathan Anderson.
Actor Jamie Dornan (Evan Agostini/Invision/AP)
He teamed a pale blue waistcoat and red tie with a dark jacket and pin-striped trousers.
Anderson was well represented on the red carpet, with a whole host of stars sporting Loewe, including Challengers actors Josh O’Connor and Mike Faist, The Bear star Ayo Edebiri, Past Lives actress Greta Lee, music superstar Ariana Grande and Waves actress Taylor Russell.
Ariana Grande in a look by Irish designer Jonathan Anderson for Loewe (Matt Crossick/PA)
Irish designer Seán McGirr also got a show of support at the Met, with Lana Del Rey sporting one of his looks for Alexander McQueen.
The Video Games singer attended the event with the creative director, wearing a nude corseted dress embellished with hand-embroidered sequinned branches.
She wore a headpiece made up of real hawthorn branches, with swathes of see-through tulle covering her, and a red rose on her hand.
Sean McGirr, left, and Lana Del Rey attend The Metropolitan Museum of Art’s Costume Institute benefit gala (Evan Agostini/Invision/AP)
The outfit referenced a look from the autumn/winter 2006 Alexander McQueen collection, which had similar tulle around the model’s head, but held up by ‘antlers’.
McGirr wore a black suit with a satin shirt teamed with black shoes with dramatic oversized leather tassels.
Meanwhile, the “Sleeping Beauties: Reawakening Fashion” exhibition showcased Wexford designer Richard Malone. Two of Malone’s 3D sculptural pieces are on display.
London-based Malone was commissioned by chief curator Andrew Bolton to create one-off pieces for the exhibition. The pieces will go into the archives of the Metropolitan Museum of Art.
The designer said: “They commissioned three sculptural pieces. They were all naturally dyed, the inside with wool and linen and the outside in recycled polyester so it is very true to my research pieces and I am very excited .
“There will only be one of each ever made so I think it is important that they have them in their collection. It’s nice because they haven’t requested that I gift them. They have commissioned them so it feels like a real invitation. I think it’s the first time they’ve done that, certainly with an Irish designer.