Ireland uses single-transferable vote (STV) for pretty much all elections.
I wonder if this voting system weakens party discipline, since candidates with strong local brand can theoretically quit the party and still get elected on personal appeal.
In response to this issue, do political parties in Ireland have means of disciplining members for not following party line?
There is not much evidence for it. In particular there is a high level of discipline to the Irish political parties. (Source)
That study contains a short analysis of factors that may affect the level of discipline. It finds that single v. multiple candidate constituencies has little to no effect (r^2 = 0.01) which was not significant at the 5% level.
While an politician with a strong local brand could stand again, he or she would then face the organisation and money of a party machine. It is clear that the threat of deselection remains a significant one for members of the Dáil. The mere loss of the whip can affect the TD's access to a promotion, to an office in Dublin, to travel perks, to research assistants, and to a social and convivial structure within political society.
The author of the above study does note that PR-STV "reduces the role and usefulness of political parties for candidates". And this does lead to TDs spending more time building a "local brand" than their equivalents in the UK or other European countries. But other reasons for following discipline are stronger, and the parties are, for the most part cohesive, certainly in comparison to, for example, the US Congress.
