issuing them with union membership forms at work, this prevented them or distracted them from carrying out their normal duties in full
I might be wrong but when one is "at work" during duty time one is usually contrated to exclusively do the work of one's employer and is usually excluded from doing work for another body.
Doing work for BALPA (or any other organisation) in work time where such activities were not agreed with the employer would be a breach of contract.
Why did this Captain not realise this and confine such activities to outside work time?
As for the "safety" issue - most airlines (and I expect Ryanair also) have a sterile cockpit requirement when below 10,000 in the last 1,000ft of climb or descent and when moving on the ground. I bet that every pilot could be pulled up on safety grounds under this requirement.
Nothing against the poor Captain involved but people need to put their brain in gear and go about things the right way to get the result they desire.
Regards,
DFC