TII said that separate proposals by Cork City Council and Cork County Council to name the south link dual carriageway Bóthar Chumann na mBan would threaten public safety.
The 15km route, the N40, is on the main road from Cork City to Killarney and Tralee.
It extends from the Dunkettle interchange, at the Jack Lynch Tunnel, to the Curraheen interchange, close to Ballincollig.
County councillors were surprised at the decision, saying the application had been made to TII as part of the centenary commemoration of the 1916 Easter Rising. Former lord mayor of Cork City Council, Mary Shiels, had previously secured City Hall backing for a similar proposal. Councillor Gobnait Moynihan said she was taken aback by the TII’s refusal.
A motion in County Hall to have the road renamed had been tabled in 2015 by her brother, Andrias Moynihan, before he had been elected to Dáil Éireann. She secured his vacant council seat.
TII officials said renaming the road would involve the costly replacement of signs and would undermine signage policy in the area.
It would also have safety implications, and many people, particularly tourists, would not understand what Cumann na mBan means. The renaming proposal “would have no relevance whatever to the road network”.
Fianna Fáil councillors Gillian Coughlan and Frank O’Flynn also expressed surprise at the TII rejection, while their colleague Christopher O’Sullivan, added: “They’re the forgotten people of 1916 and this would be a massive gesture to name the road after them.”
However, council chief executive Tim Lucey said he would also have concerns about renaming the road.
“I think there’s a more appropriate way of honouring Cumann na mBan,” he said.
Ms Coughlan suggested that the route, the N40, could remain on road maps and GPS systems and she could not see a problem in putting up several Cumann na mBan signs along the road.
Mr O’Flynn, who is head of the council’s 1916 commemoration committee, said the committee would further examine the situation and possibly look at other ways of commemorating the women.
It is unclear if the TII decision will have any ramifications for a renaming request in West Cork. The council has asked TII for permission to name a roundabout in Clonakilty after the late Joe Walsh, a former minister for agriculture.