While Junia has received a lot of attention, Mary (Rom 16:6) has gone almost unnoticed. She is mentioned ahead of Andronicus and Junia, which seems surprising since Paul gives no indication of personal links with her. If she had been an important co-worker of Paul or his close friend, he surely would have mentioned it. So it seems to me that her position in the list, ahead of Andronicus and Junia, is explicable only if she was very well known and respected by the Christians in Rome. Paul's audience would, of course, have known of her prominence in their church, so Paul would have had no need to mention it. He says only that she has worked hard in Rome, and I think he is saying here that she deserves the respect that accompanies her leadership role in the church (compare 1 Cor 16:16).
Tal Ilan (Lexican of Jewish Names in Late Antiquity part III p64) counts 23 Jews with the name Mary/Mariam/Marian in the western diaspora, out of 552 females. This amounts of 4.2%. In Palestine, on the other hand, (part I p55-56) 70 Jews had the name, out of 317 female Jews. This amounts to 22.1%. Therefore the name was about 5 times more popular in Palestine than in the western diaspora. This observation, which has been overlooked by the commentators, suggests that she had moved to Rome from Palestine. She may have introduced the Christian faith to Rome, and this would account for her prominence there. It matters little whether the Latin name Μαρίαν or the Hebrew Μαριάμ was the original. A Μαριάμ could easily have adopted the name Μαρίαν after moving to Rome.
In summary, the Mary was the most prominent member of the church of Rome known to Paul.
Commentators have differed greatly in their understanding of her. Origen patronized her:
Paul is teaching here that women too ought to work for the churches of God. They work when they teach childreen how to behave, when they love their husbands, when they feed their children, when they are modest and chaste, when they keep a good household, when they are kind, when they are submissive to their husbands, when they exercise hospitality, when they wash the feet of the saints, and when they do all the other things which are allotted to women in the Bible. Commentary on the Epistle to the Romans. CER 5:248Chrysostom gave her a teaching role (Homilies on Romans NPNF 1 11:554), but in 1879 William Shedd denied her such a role. Olshausen (1849) and Dodd (1932) did not mention her at all! In 1994 MacArthur suggested that she was a founding member of the church of Rome. Osborne (2004) concludes that "she did play an important role in the life of the church" of Rome. Jewett (2007) writes, "Miriam functioned as an evangelist in Rome". It would seem, then, that commentators are beginning to recognize this Mary's importance.