Orion will pay $1.47 per share, a 62% premium to Wednesday's closing price, with certain members of Dalradian management, Sean Roosen and Osisko Gold Royalties to retain their combined 20.4% stake.
Dalradian shares spiked 59% in Toronto Thursday, capitslising the company at $515.5 million.
Dalradian owns the high-grade Curraghinalt gold development project in Northern Ireland.
Orion chief investment officer Oskar Lewnowski said Orion was pleased to be acquiring one of the world's best undeveloped gold deposits.
"Curraghinalt gets more exciting as additional exploration and engineering work is completed, the recent positive resource update being just the latest example," he said.
"The Dalradian team has done an outstanding job of growing the deposit and advancing it through the feasibility stage."
Dalradian CEO Patrick Anderson said this week Orion and Osisko were showing "great confidence in Northern Ireland, our high grade Curraghinalt project and our West Tyrone workforce".
"Their goals as strategic investors are completely aligned with ours - to secure planning permission for an underground gold mine and continue advancing the project in a way that benefits all our stakeholders.
"We will continue to work closely with community members, all levels of government, employees, contractors and local suppliers toward building one of the world's top gold mines."