The Friends of Connacht Rugby has handed in a letter to IRFU headquarters in Dublin protesting the body's plans to end the province's professional status.
Around 1,000 people marched on Lansdowne Road to oppose the IRFU's proposed budget cuts.
Despite the demonstration, IRFU chief executive Philip Browne said he has little option but to press ahead with the plans.
"Professional sport is struggling in a number of areas against a stagnant commercial backdrop," he said.
"It's an issue that we're going to have to address - we simply can't live beyond our means."