More than 120,000 landlords could face fines of €3,000 after failing to register their properties by the December 1st deadline.
Figures from the Department of the Environment revealed just over 20,000 forms had been received by the Private Residential Tenancies Board.
Ms Aideen Hayden, chairwoman of Threshold housing campaign group, said compliance would only come about if the board actively chased landlords.
"There's an onus on the board to send a clear message to show that the legislation will be actively followed," she said.
Ms Hayden said it was possible they were playing a wait-and-see game. But she warned that non-compliant landlords would be tracked down, pursued through the courts and fined up to €3,000 for failing to register.
Landlords previously had to register with their local authority, but less than 20 per cent of the 141,000 landlords identified in Central Statistics Office surveys registered.
It is thought the final total may be as low as 30,000, or one fifth of the total number of landlords.