TARLACH MacNIALLAIS  born Terence Nellis in Belfast, Northern Ireland, on this date (d: 2020), the tenth of 11 children of John James Nellis, a bus conductor, and Una Nellis, a homemaker. He later adopted the Irish language version of his name.

As a university student, he protested against British imperialism and emerged as a gay rights advocate. He responded to the “Save Ulster From Sodomy” campaign — an ultimately unsuccessful effort to keep homosexual acts illegal in Northern Ireland — by helping to mount a counter campaign with the slogan “Save Sodomy From Ulster.”

He moved to New York in the mid-1980s and became involved in the protracted struggle by L.G.B.T. groups to be fully included in the St. Patrick’s Day Parade up Fifth Avenue. Many years later, he became a member of the New York City St. Patrick’s Day Parade formation committee, and marched in the parade with the Lavender and Green Alliance in 2016.

Brendan Fay, a friend and fellow activist, emphasized that Mr. MacNiallais’s well-publicized activism for inclusive parades should not overshadow an equal passion. “Most of his life in this city was advocating for people with disabilities,” Mr. Fay said.

He was known for his decades of advocacy for L.G.B.T. and disability rights. “A battering ram on issues of importance,” according to Harriet Golden, a vice president at A.H.R.C. New York City, an organization that serves people with intellectual and developmental disabilities, where Mr. MacNiallais worked for nearly 35 years.

MacNiallais died on April 1, 2020 due to complications of the coronavirus, according to friends and family.