Well done mise le meas- its very hard to find info on this. There is also film footage of the dumping so I’ve been told.
This has been raised in the Dail 1995 below is excerpt and link. http://debates.oireachtas.ie/dail/1995/03/23/00003.asp
Dr. McDaid: I seek leave under Standing Order 30 to move the [2] Adjournment of the Dáil to discuss a matter of grave importance, namely, the fact that the British Ministry of Defence has admitted for the first time that hundreds of thousands of tonnes of chemical weapons and poison gas were dumped off the Donegal coast. It has admitted that 3,500 tonnes of 23 pound artillery shells containing mustard gas, tabrun and sarin nerve gas were dumped in sealed containers — that is, one foot of concrete — over the side of British vessels about 30 miles off the Donegal coast. This is but the tip of the iceberg in terms of the information that is available to me.