In fact he investigates him quite good, collectin empty bottles after their drinking tour, talking about Irish prisoners and he's almost clearly saying that Devlin was once in prison because of his activities and was left out to continue with his "work".
Recall Columbo's final limerick, in
the duel of limericks he had with
Devlin:
"There once was a man named Finnegan,
who broke out of jail so to sin again,
he broke laws by the dozen,
even stole from his cousin,
so the jail he broke out of, he's in again!"
That, of course, is Joe Devlin's life story, so Columbo is telling him he's on
to him ("stealing from his cousin" is the
money he is taking under false pretences from his "Aid To Northern Ireland Committee" which is supposed to help the widows and orphans but is actually going
to the paramilitaries.
The "Nicest" villain may be hard to choose,but the "Worst" ones got to be from this episodes:
1.Lovely but lethal
2.Lady in waiting
3.Most Crucial Game
4.Publish or Perish