Dating for eight years, the That 70s Show actor, 35, and Almost Famous starlet, 31, tied the knot at an Irish castle in front of a small group of friends and family.
Model-actress Phillips (the daughter of the Mamas and the Papas' John Phillips and the half-sister of Chynna and Mackenzie Phillips) wore custom Zac Posen, while Masterson suited up in Albert Hammond Jr. for Confederacy.
Danny's brother Chris served as best man, with high-profile guests that included actors Ben Foster, Mike Pena and Ethan Suplee.
One A-list pal not in attendance was Masterson's That 70s Show costar Ashton Kutcher. Kutcher, 33, helped celebrate his friend's nuptials with a wild bachelor's weekend in San Diego Sept. 23 - 24. A source confirms to Us Weekly that Kutcher, the Masterson brothers and other friends were in San Diego -- partying at danceclub Luxx and the Hard Rock Hotel -- to celebrate Danny's upcoming wedding.
As reported in Us, it was an early-morning afterparty at Kutcher's suite at the Hard Rock that he met local girl Sara Leal, with whom he had a torrid one-night stand.
Kutcher told Leal and other young women at the bash that he was separated from wife of six years Demi Moore.
"I'm a normal 22 year-old girl, and I spent the night with someone who I thought was separated," Leal later told Us Weekly. "If I had known he was happily married, none of this would have ever happened."