When the MacManuses leave the police station, one of them calls a number from a phone in the station and is informed of a location, presumably that of the Russian mafia meeting. Who is the informant? What was his motive?
It's been a while since I've seen the movie but they are informed through a pager with the info.
They got the pager from the Russian mobsters who tried to kill them at their flat (after the bar fight). After the interrogation with the FBI agent, where we learned that they speak a wide variation of languages fluently (Russian included), they get a phone number paged on the russian pager.
One of the MacManuses calls the number and they get the adress and the hour of the meeting etc.
As it is stated on Wikipedia
Connor learns of a meeting of Russian syndicate bosses at a hotel from a pager taken from one of the dead Russian mobsters.