“We are going to look back at the mid-2010s as the high-water mark of free speech online, and it is only downhill from here,” said Eric Goldman, a law professor at Santa Clara University and a champion of those legal protections.

If a wave is building, Mr. Tweed is positioned at the weakest point in the breakwall. European regulators and lawmakers have been far tougher on the technology companies than has Washington, where the industry flexes more lobbying muscle. And Mr. Tweed practices out of offices in the especially favorable jurisdictions of Belfast, London and Dublin.

Irish law is much stricter against defamation than that of either the United States or Britain, while British law is much stricter about protecting personal privacy. In 2013, Northern Ireland rejected a measure passed by the British Parliament to deter foreign plaintiffs from so-called libel tourism, making the Belfast courts uniquely welcoming to Mr. Tweed and his American and international clients.

In the past, he has filed lawsuits in Belfast courts against The National Enquirer on behalf of Ms. Spears and the singer and actress Jennifer Lopez (over false reports of marriage trouble for one and of a drug scandal linked to the other). In London, he sued the MailOnline website on behalf of the actor couple Ashton Kutcher and Mila Kunis (over paparazzi pictures of their baby on a beach in Santa Monica, Calif.). And in Dublin, he has sued the German magazine Heat on behalf of Mr. Timberlake and his wife, Jessica Biel (over false suggestions of trouble in their marriage).

Like most of his celebrity cases, all those claims were settled out of court.

With a mane of wavy blond hair, open-collared shirts and a manner other Belfast lawyers describe as “impish,” Mr. Tweed often says he feels as much at home in Los Angeles as in Belfast. But in recent years, he said, he has found a growing number of less famous clients also seeking to protect their reputations from embarrassment or insults on social media, or from threats to post the information.

“A threat used to be somebody leaving a pig’s head in a bed,” Mr. Tweed said. “Now it is ‘I am going to put this on Facebook or Twitter or YouTube or Periscope!’ ”