There's been a substantial increase in support for Sinn Féin over the past month according to a new opinion poll.

The RED C survey for tomorrow's Sunday Business Post also sees the coalition parties lose some of their recent gains.

Both the Yes and No votes in the same-sex marriage referendum are down, with an increasing number of people undecided.

In the past month support for Fine Gael and Labour has dropped two points each to 25 and 8% respectively.

Fianna Fáil goes up one point to 19%, with Sinn Féin gaining five points to 22% - recovering all of their losses a month earlier.

Independents and Others are down two to 26% - the Greens and Renua Ireland are included there on one percent each.

The poll - conducted on Monday, Tuesday and Wednesday of this week among 1,006 adults - also assessed those in favour and against the same-sex marriage referendum.

Support levels here compared to a RED C poll six weeks ago, show the Yes at 72, down slightly, No at 20 also down slightly and an increase in don't know's to 8 points.

One in three of likely Yes voters also expressed some reservations.