The BBC reported that the mother of the young girl saw "a bomber leave what was the second bomb beside their shop."
There was no identification given.
http://www.bbc.co.uk/news/uk-northern-ireland-11026426

24 Aug 2010.

Another BBC article, also dated 24 Aug 2010, states:
"The sudden death of Fr Chesney in 1980 means he is not able to defend himself. The failure to arrest him meant he never got a chance to tell his side of the story.
"Although the police had a huge file of intelligence information linking him to terrorism, they did not seem to have much hard evidence.
http://www.bbc.co.uk/news/uk-northern-ireland-11071321

The investigation can point to errors in investigation and procedures, but seems not able to unequivocably condemn the priest.
Major participants are dead, so their reasoning cannot be definitely determined.