Since Irenaeus is the first witness to the Book of Acts and he used it to discredit the Gospel of Marcion, the simplest explanation is that Irenaeus wrote, or had someone else write, Luke-Acts, using the Gospel of Marcion as framework and perhaps other sources, for example a different genealogy for Jesus (than the one in Matthew). The purpose was to discredit Marcion. Marcion actually wrote the Gospel of Marcion. But Irenaeus claimed Marcion edited Luke-Acts to produce his gospel. Thus Irenaeus was able to claim that Marcion was a fraud, by claiming that he butchered a previous apostolic work, written by Luke the Evangelist who was claimed to be a physician of Paul's. Acts discredits a Marcionist interpretation of the Early Church and Paul, by attaching it to an alleged Luke, i.e. Luke-Acts, Irenaeus was able to make it appear pre-Marcion and thus discredit Marcion.