Different rules apply to by-elections. They can be a bit of a lottery, with votes giving governments mid-term kicks or giving a chance to a quirky, celebrity or anti-establishment party.
For a thirty-year period in Irish politics, no Government party succeeded in winning a seat in a by-election. The Labour Party broke that pattern in October 2011 when Patrick Nulty won the seat left vacant by the death of Fianna Fáil's only TD in Dublin Brian Lenihan.
And now Nulty himself, who later severed his connection with the Labour Party, is vacating the seat after admitting sending inappropriate messages to a 17-year old girl facebook.
It's an unusual situation having a second by-election in the same constituency in the one Dáil term (although it happened in Tipperary South in the 1997 to 2002 period).
And while the first by-election happened within a year of the Coalition coming to power -and before it had its sheen - it still provides something of a form guide for how the next poll will bear out.
Within minutes of Nulty’s resignation, Fianna Fáil’s young councillor from Corduff David McGuinness was installed as the favourite. He came second in the by-election in 2011 against all the odds (Fianna Fáil’s lot had tanked in the capital at that stage) garnering an impressive 21.7 per cent of the vote.
That said, the Socialist Party candidate Ruth Coppinger was not all that far behind McGuinness, on 21.1 per cent of the vote. Indeed both candidates were tied after the fourth count with McGuinness surviving on the basis of having a higher percentage of first preference votes.
The constituency, taking in most of Dublin 15, is split almost evenly between middle class and working class areas. In the next election the expectation is that the four seats will split evenly between Fine Gael, Fianna Fáil, and two-left wing seats, probably the Labour Party's Joan Burton and the Socialist Party.
Fine Gael have developed a strong presence in the constituency through Leo Varadkar. However, its candidate in the byelection, veteran councillor Eithne Loftus, was not considered strong and she made little impression. There has been talk that Fine Gael will parachute in a big celebrity to stand here - think Eamon Coghlan or Kenneth Egan.
But the party doesn't have the potential to dominate here as it does in South Dublin and it's likely that the public will be far warier of celebrity candidate stunts after the George Lee experiment in South Dublin in 2009. Its likeliest candidate could be Kieran Dennison who stood in the general election in 2011.
I don’t think there will be a huge queue forming to become the Labour Party candidate, given that Nulty stood for the party and then left. Allied to that is that support for the party has fallen significantly. This is not 2011. And the chances of the party winning a seat is remote.
Sinn Féin have not developed the same profile or presence in this constituency as it has elsewhere. But things don't stand still. Paul Donnelly got 9 per cent of the vote in 2011 and will probably get a greater share this time but not enough to be in with a shout. This election will be about consolidation, working to win a seat here in the medium term.
One of the other surprises in 2011 was the comparatively strong showing of the Green Party’s Roderick O’Gorman, with 5 per cent of the vote. He has been a constant presence in the constituency and could lay down a marker for the future.
Still, it’s going to boil down to the big two. There is little doubt that McGuinness will top the poll but will he stay sufficiently ahead of Coppinger to take the seat. It’s clear that Fianna Fáil may attract a stronger first preference vote now. But remember also that Nulty was very much on the left wing of the Labour Party and some of those who supported him in 2011 may be minded to switch allegiance to the Socialist Party, or its nom de guerre, the Anti Austerity Alliance.
Remember also that in the 1996 byelection, Joe Higgins ran Brian Lenihan junior very close in the contest for the seat left vacant by Brian Lenihan senior. The margin between both candidates in first preferences was small and Lenihan managed to keep his nose in front.
At this moment in time, that’s the pattern that looks most likely to occur in this byelection.