In the past I have felt that events of Galatians 2 were contemporaneous with the events of Acts 15. In recent further study I have come to believe that in timing they better comport with Acts 11:29ff. Here are some brief comments on the subject.
The differences between what Luke writes in Acts 15 and what Paul writes in Galatians 1-2 are differences in fact, I have concluded, because the visit to Jerusalem mentioned in Galatians 2:1ff does not correspond to his visit to Jerusalem in Acts 15.
Comparing and contrasting Acts 15 and Galatians 2
Acts 15 Galatians 2
Third visit (Acts 9:26; 11:30,12:25) | No third visit mentioned
Problem, reason for going (15:2) | Problem after arrival (2:3-5)
Sent by church (15:2-3) | By revelation (2:2; cf. Acts 11:27-29)
Public discussion (15:6ff) | Private meeting (2:2)
Circumcision and salvation (15:1,5) | Table fellowship (2:11-13)
Focus on James’s resolution (15:19) | James’s resolution unmentioned
Paul a minor participant (15:12) | Paul a primary participant (2:1-9)
By the time of Acts 15 in the historical record of the book of Acts, Paul has visited Jerusalem three times – the first visit is mentioned in Acts 9:26ff, the second is to bring famine relief from Antioch to Jerusalem and Judæa (Acts 11:27-30), and the third is for counsel with the Jerusalem Church on the matter of circumcision and the law of Moses (Acts 15).
“I went up again” in 2:1 seems to be a reference back to a prior visit to Jerusalem mentioned in Galatians 1:18. Without a presupposition in favor of Acts 15, the comparison of 2:1 and 1:18 would normally and simply be taken as the next (therefore, second) visit Paul makes to Jerusalem. (That is, “I went up to Jerusalem” in Galatians 1:18 is the first visit, and “I went up again to Jerusalem” in Galatians 2:1 is the second visit.) The second visit of Paul to Jerusalem, according to the book of Acts, occurs at the time recorded in Acts 11:29-30, 12:25. The events of Galatians 2:11-21 strike an odd chord if they track with Acts 15 or follow after the conclusion of the consultation. The influence of the Judaizers in Galatians is much more understandable if occurring before the decision of the church in Acts 15 rather than after it (cf. Acts 16:4). In Galatians 2, Paul calls the opposers “false brethren.” In Acts 15, Luke calls the opposers “Pharisees which believed.” Also, Galatians 2:10 “remember the poor” is more consonant with the purpose of the Acts 11 visit than the Acts 15 visit. This further better explains why Paul does not refer the Galatians to the decision of the consultation in Jerusalem – it had not taken place at the time he was writing to the Galatians.