Campaigners are set to go to the High Court to legally challenge the use of three seat constituencies in areas of low population.
The new constituencies of Sligo-North Leitrim and Roscommon-South Leitrim were proposed by Fianna Fáil and Fianna Gael in submissions to the independent body responsible for redrawing boundaries of Dáil Éireann constituencies, the Constituency Commission.
The lobby group opposing the split, the Save Leitrim Campaign, says it will now pursue a legal challenge to the proposed division which the Government plans to implement.
The campaign believes the division of the county will deprive Leitrim of the chance to elect a local TD in future elections as it believes all three TDs could be based in the Co Roscommon end of the constituency.
Expressing disappointment at the Government decision to proceed with the split, despite widespread concern within the county, the campaign has collected 15,000 signatures on a petition opposing the move.
The campaign can solely challenge the Leitrim changes, but are more likely to take a landmark case on the use of three seat constituencies in low population areas.
After the campaign heard detailed legal advice on how to proceed with a court challenge, Save Leitrim Campaign spokesman Cormac Ó Suilleabháin said at least 100,000 would be needed to pursue the case.
"The use of a three seater militates against getting a TD elected in an area of low population," he said.
Documents in the Department of the Environment, viewed by the Irish Examiner, show clearly that the change had the imprimatur of FF and FG.
The Fianna Fáil headquarters' submission stated: "Sligo-Leitrim is no longer a viable constituency; however, when these two counties are combined with the contiguous county of Roscommon, there is the basis for six members of the Dáil in two three-member constituencies, Sligo-North Leitrim and Roscommon-South Leitrim.
The Fine Gael headquarters' submission stated: "As neither County Sligo nor County Roscommon has enough population to warrant three seats, we propose that two new three-seat constituencies be created those of Sligo-North Leitrim and Roscommon-South Leitrim."
While the Constituency Commission subsequently adopted the suggested changes, they have since been criticised by Leitrim-based members of both parties.
The Government approved the report of the Constituency Commission last month and Minister for the Environment, Dick Roche, is currently preparing legislation to introduce the recommended changes.
Although the Commission said the membership of Dáil Éireann should not be increased beyond the existing level of 166, it did recommend a net increase of one in the number of constituencies, from 42 to 43.