A new political opinion poll has delivered Sinn Féin its largest ever setback in a single month.

The Behaviour and Attitudes poll for The Sunday Times shows the party down six points to 17%.

This is their lowest result since Mary Lou McDonald took over as party leader.

Ms McDonald has also seen her personal satisfaction rating slipping back four points to 44%.

The poll was conducted among 911 Irish voters, from December 6th to 18th.

Fine Gael is up one to 31%, Fianna Fail up two to 29%, and Labour is down one to 5%.

The Taoiseach Leo Varadkar's personal satisfaction rating is unchanged at 48%, while Fianna Fáil leader Micheal Martin pulls ahead up one to 50%.

Taoiseach and Fine Gael leader Leo Varadkar remains unchanged | File photo

Political editor with The Sunday Times, Stephen O'Brien, says it has been a difficult few months for Sinn Féin.

"The abortion issue was difficult for the party - Fine Gael and Fianna Fáil gave their Oireachtas members a free vote, a kind of a conscience vote.

"Sinn Féin didn't do that: Sinn Féin stressed that they are a party which votes to decide the party position and that the majority view holds.

"And then they lost Peadar Tóibín, a very able TD from Meath-West, and Carol Nolan, a first time deputy from Offaly.

"They lost both of those over the abortion issue".

"Of all the parties, Fianna Fáil will be happiest with this poll - it's up another two points to 29%, and therefore comes within two percent of Fine Gael.

"That's a narrow gap and it's the closest Fianna Fáil has been to Fine Gael all year.

"It's the highest rating Fianna Fáil has enjoyed in the polls all year, so it's a good result for them".

Fianna Fáil leader Micheal Martin has seen his popularity grow | File photo

On party leader Micheal Martin's popularity, O'Brien says it is "possibly a reward for Micheal Martin's responsible declaration - and kind of a unilateral and unexpected declaration - that he would not seek to bring the Government down until the Brexit question had been resolved.

"The voters seem to have connected with that, it certainly seems to be one of the factors feeding into this period of recovery that Fianna Fáil has enjoyed over a few months now".