DFC,
You Say
"IAA recruited people into the ATC assistant grade. In time they did a tower course and worked in the tower only until a vacancy came up on area procedural / approach radar (15 years doing tower only not being uncomon) "
Speaking from the shannon Perspective many of these people were left in the training grade for up on 15 years through no fault of their own. All that was required was a rating course and a period of on the job training, problem solved.
Instead they spent about a third of their career in a training grade not being trained and wages to match.
If you want to understand the political motivation of this controller shortage then ask IAA management why they had a significant number of staff who had completed more than half of their controller training requirements, had years of experience with their respective units but were not progressed to controller grade.