The Minister for Education has blamed teachers unions' "last out first in" approach for inefficiencies resulting in teacher shortages in some areas.
He made his criticism when he announced 300 additional teaching posts in the special needs area from September.
Mr Dempsey made the announcement as a row brewed between his Department and the ASTI concerning the decision to withdraw 300 "concessionary" teachers from September.
The ASTI accused the Department of Education of "neglecting to clarify" the additional 300 teaching posts he announced for September are in the special needs area.
This will have no bearing on the ill effects of the reduction of the concessionary posts from 1,000 to 700 after the summer.
Fine Gael accused Mr Dempsey last night of "doubling back on his pledge to promote science in schools".
Labour education spokesperson, Deputy Jan O'Sullivan, said that Mr Dempsey was scapegoating concessionary teachers and called on him to provide "immediate clarification on the fate" of the 300 concessionary teachers being dropped.
The Green Party demanded a reversal of this latest example of "voodoo economics".
Concessionary teachers fill in when a school wants to keep a wide subject choice, but has suffered a demographic drop in pupil numbers. The school needs to hire in part-time teachers to fill in on minority subjects.
Most of the 300 additional posts, announced yesterday, will be involved in teaching support to help schools meet the needs of special needs pupils, the Department confirmed. It is not possible to say yet which secondary schools will receive additional resource teaching hours.
"Allocations are taken on an ongoing basis as assessment reports are received and considered," the Department stated.
The Minister for Education's claim that there will be 300 additional secondary teachers in September is "inexplicable", claims the ASTI.
A "great surprise" was the reaction of Mr George O'Callaghan, general secretary of the Joint Managerial Body which manages the majority of secondary schools.
The need for concessionary teachers arises because in most secondary schools, teachers cannot be transferred from one school to another. In comparison, the VEC sector has a more flexible approach, in which teachers can be redeployed as needs arise.
The Minister stated: "There is considerable inflexibility in redeploying over quota teachers other than in the VEC sector. Unions have traditionally insisted on 'a last in first out' approach. Very often the surplus teacher is not the most junior in the school. If there were a more effective redeployment arrangement, over quota posts (some 320 in all) could be targeted where needed."
A lack of redeployment means that some schools are overstaffed while others lack teachers in essential areas, Ms O'Sullivan agreed.
She would like to see teachers being encouraged to move where they are needed, while being able to retain their current level of salary.
The "first in last out" approach means that if a teacher changes schools, then he or she starts again at the bottom of the salary scale.