The Association of Secondary Teachers Ireland (ASTI) goes into its annual conference today having lost almost 450 members in the first three months of the year, and with its own staff in revolt over an unresolved internal dispute.
The secondary teachers' union has been embroiled in controversy on several fronts for a number of years, with no sign of resolution.
The ASTI is refusing to co-operate with junior cycle reforms, and it is the only public service union not to have accepted the Lansdowne Road Agreement (LRA) on post-austerity pay restoration.
The most recent ballot on these proposals, the results of which were announced in February, showed a divided union, with a narrow 52.5pc-47.5pc vote against acceptance.
As the disputes dragged on, there has been anecdotal evidence of ASTI members resigning, although that was not borne out by membership figures of 18,300, posted in January.
A recent meeting of the ASTI executive committee was told that 446 members had resigned in January, February and March, most of them in March - a level that would be regarded as unusual in the teaching sector in these months.
It is not known if these losses were balanced by new entrants in that period, and, if so, how many.
An ASTI spokesperson would not comment on the figures, but said membership levels fluctuate from month to month and the union only does an official count, annually, based on membership on December 31.
The ASTI conflicts have closed schools due to strike action and disrupted the assessment of junior cycle students, while its members are losing out on improved pay and conditions because they have not accepted the LRA.
Young teachers in the ASTI are the worst affected and are currently up to €220 a month behind their counterparts in other teacher unions, and also have to wait four years for a permanent contract, rather than two years agreed under LRA.
Meanwhile, there has been no settlement of a dispute between full-time staff and the union's elected leadership, despite a reference to the Workplace Relations Commission more than a year ago.
With no progress made, Siptu has written to the ASTI on behalf of staff advising that it has decided to conduct a ballot on industrial action. There are about 20 staff involved, members of Siptu and the NUJ.
At the root of the staff complaint to the WRC in December 2015 was that the union leadership failed to protect them by not adequately dealing with a range of grievances, such as criticisms made in the media and on websites by a small cohort of ASTI members.
An ASTI spokesperson said the union endeavoured to have good relations with staff and that such issues were confidential.