The man and two women were arrested in relation to the case yesterday.
Three people held in relation to in relation to the murder of William Delaney have been released without charge.
The 56-year-old was last seen in Monasterevin in January 2019, but Gardaí were later told he had been killed and buried in County Laois.
A search of land around the Rock of Dunamaise did not lead to the discovery of his remains.
A man in his 30s and two women in their 30s and 50s have been were arrested as part of the investigation yesterday.
Gardaí say they believe people in the community have information and are yet to come forward.
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