The Pacific Islands neighbours were elected onto the Council, which now comprises 51 members, after successfully meeting new governance criteria, following reforms launched in 2015.
World Rugby Chairman Sir Bill Beaumont hailed the the landmark meeting and praised "the enormous contribution" Pacific Islands countries have made to the global game.
"This is an historic day for World Rugby and the Pacific Islands, and a reflection of the importance and success of the transformational governance reforms made by this organisation and the unions," he said.
"We are delighted to be welcoming Fiji and Samoa to Council, two unions who have contributed so much to the game. The Pacific Islands are unique, immersed in rugby heritage, and I know that the unions will bring excellent insights and make strong contributions on Council.
"This shows that the model is in place, the pathway is in place and the door is open to other unions who aspire to have a seat on Council. We encourage all unions to take inspiration from Fiji and Samoa, review their governance and strive to achieve the required criteria.
"World Rugby is committed to the sustainable growth of the sport set against a backdrop of strong governance, and we will continue to work with our unions to ensure that they have all the support to achieve and maintain the necessary criteria."
Fiji Rugby Union Chair Francis Kean said: "This is a significant moment for Fiji rugby and all those who have worked towards this objective, implementing new structures and governance pathways. We look forward to working with World Rugby and the Council to further our contribution to the global game in the board room."
The Samoa Rugby Union Chair, Prime Minister Tuilaepa Malielegaoi, added: "The World Rugby Council is a very important place to be - it is where decisions are made on the future of the sport and Samoa is very excited to be here.
"We've played an active role in participating in international rugby competitions across the world - from fifteens to sevens - despite being a very small country with a small player pool so for us to be here at this meeting, and to unite with World Rugby on their vision to grow the sport, is an incredible learning experience."
A record 17 of the 51 World Rugby Council member positions are now occupied by women including Fiji's Cathy Wong, who was one of two Oceania Rugby representatives in attendance in Dublin, alongside Papua New Guinea's Richard Sapias.