Collins was first elected to Dáil Éireann at the 2007 general election for Limerick West. He was elected on the first count, and the highest vote of any of the newcomers to the 30th Dáil. His grandfather James Collins represented Limerick West in the Dáil from 1948 until 1967. His uncle Gerry Collins is a former Minister and MEP, who was a TD for Limerick West from 1967 to 1997. Another uncle, Michael J. Collins, sat for Limerick West in the Dáil from 1997 until he retired in 2007.
In February 2023, political news website The Ditch published a story claiming that Collins had misled Limerick County Council about his place of residence in a 2001 planning application.[3] While not addressing the question of stated residence, Collins said he was "entirely satisfied" that his planning application was valid, and that he met the correct planning criteria applicable at the time.[4] He told the Dáil that the article in The Ditch was "misleading and inaccurate".[4] The Tánaiste Micheál Martin defended Collins, saying that he was entitled to seek planning permission.[5]
In April 2023, a further investigation by The Ditch found publicly available information that showed that Collins's wife, Eimear O'Connor, a GP, requested to buy land from Limerick County Council in 2006.[6] Following a motion by fellow Fianna Fáil councillor Leonard Enright, Niall Collins voted at a local area committee meeting in favour of selling the land. The sale to O'Connor was formally approved in 2008 at a meeting of Limerick County Council, for which the minutes included "The disposal of this site was agreed by the members of the Bruff Electoral Area at the meeting held in January 2007", in which Collins had participated;[7] Collins had been elected TD in May 2007 while the sales process was proceeding.[8] Under the Local Government Act, 2001, a county or city councillor is prohibited from voting when "where he or she has actual knowledge that he or she or a connected person has a pecuniary or other beneficial interest in, or which is material to, the matter".[9] In December 2007, Eimar O'Connor sought planning permission to build a medical centre and offices on the land, which she had yet to purchase. This permission was given despite objections from local residents, who said in a letter "local school children in the nearby primary school regularly use this green space". O'Connor became owner of the land in January 2009. Trees and hedging were cut down, and the site left vacant for the following decade.[10] In April 2020, an email was sent to the County Council looking to discuss selling social housing to the Council.[6] Further to this, a planning consultant had claimed that Collins was the owner of the land, a claim Collins denied.[6]