In Northern Ireland should a councillor (or for that matter a MLA) die or resign his seat there is no by election but a co-option by the party that held the seat. That change for Councillors came about in 2010.
replace the late Tony Hill in Bangor West. Hill had been the Alliance Councillor for 11 years since 2001, Harbinson hadn't even managed 8 months before this morning announcing he was resigning from the party while at the DUP conference. He has not said that he is changing allegiances, merely thinking about it.
Earlier this month Harbinson was the one Alliance Councillor here who had voted against the marriage equality motion before council. He was entrusted with the seat on behalf of the Alliance Party and now less than a year on he trod all over the legacy of that faithful and dedicated party man he replaced.
There is no mechanism that can make Adam Harbinson resign his seat. Although as he himself does not have a mandate and is co-opted on behalf of the Alliance Party and vote for Tony Hill I believe he should do the honourable thing and resign his seat on council as well as resigning from the party, otherwise he is stealing the seat from both. He probably should have done that in the other order seat first and then party though.