The Northern Ireland Code of Local Government Conduct, which is subscribed to by councillors, offers the following guidance when there's a choice to be made between the interests of the public and the party (or some other sectional interest):
10. You should assist your council to act, as far as possible, in the interests of the whole community. Whilst individuals are entitled to pursue their own personal concerns about local issues you are not obliged to represent their views above the wider public interest.
Even when there's a problem involving sincerely held principle, there's still a need for councillors to exercise discretion, especially when they are representing the whole community as well as the council.