Labour TD Aodhan O Riordain, who is leading the campaign against housing development at St Anne's Park, proposed planning changes to allow residential development on the land.
Mr O Riordain was a member of Dublin City Council in 2005 when he and his colleagues called for residential development use on Z15-zoned, institutional lands. This includes the St Anne's site.
However, in recent years he has spearheaded opposition to plans by Pat Crean's Marlet Group to develop on the St Paul's College land.
Mr O Riordain and local campaigners hope for a judicial review against An Bord Pleanala's decision last month to allow more than 650 homes there.
Last week, Marlet wrote to Mr O Riordain claiming his role in rezoning Z15 lands to include residential use "appears significant".
The letter, signed by Mr Crean, claimed that the newly-elected TD's involvement appeared to be "entirely at odds" with, and "make a mockery of," his position as an opponent of the development.
Minutes from a council meeting held on January 31, 2005, show a motion by Labour Party councillors, including Mr O Riordain, calling for residential development to be allowed on Z15 sites.
It was a proposal that received cross-party support and the Labour motion was ultimately withdrawn.
Last night Mr O Riordaintold the Herald that he did not represent the area at the time and hit out at the developer for "playing the man not the ball."
"I received a bizarre correspondence from the developer over the weekend. I consider it quite inappropriate. The democratic process is there for people to engage with it," he said.
"I support the democratic process, I don't trust developers.
"He is in it for profit, that's the capitalist society we live in, I'm not in it for any profit."
Asked about his motion to allow residential development on the land, he said Z15 zoning is "clearly for institutional use," such as by a religious order.
He said An Bord Pleanala is not forced to adhere to zoning as strictly as the council.
Mr O Riordain said local opposition groups have sought legal advice about a challenge.
"We only have a number of weeks, so time is of the essence. It's not an inexpensive decision to make so we want to make sure we're on a solid legal footing."
The appeals board has given Mr Crean's Marlet Group the green light in spite of strong local opposition against the plan.