Billy Costigan's uncle Jackie is mentioned in The Departed with Queenan and Dignam, then again during Sullivan's briefing scene. How was he important to Frank Costello's crew?
It's never mentioned what his exact role was. All we know is that he was working for Costello as a probably not too important but loyal mobster.
We learn that he probably was no stranger to violence when Costello approaches Billy:
So Costello is implying that Jackie was a different man than his brother William Costigan. He spoke his mind and could be motivated with money.
A later conversation makes it clear that Jackie had earned his trust in the crew:
He got killed when Billy was still young:
(all quotes taken from IMDB)
That is pretty much all I could find regarding Jackie. He did not play a big role in the movie, the main purpose of the character was to give Billy Costigan a family connection to the mob. He is the reason Costello approached and recruited him, because he knew and trusted Jackie and saw that potential in Jackie's nephew as well.