The present letter is not dated and it can only be said that it was written at some point during the episcopacy of Ambrose. Ambrose's addresee, Iraeneus, is treated with paternal affection. He received several letters from Ambrose but is known only from this correspondence. He is never explicitely called a presbyter (or a cleric) and there is a scholarly discussion whether he actually was one - see especially Palanque 1933. That Irenaeus was a lay man is claimed by Paredi 1968: 498 and Zelzer 1978: 15. He is considered a cleric by Mazières 1979 and later by Zelzer 1990: XXI, n. 6. See PCBE, Italie 1, Irenaeus 1.
As the letter begins from the mention of a previous letter, some scholars thought that it should be attributed to Sabinus, bishop of Piacenza, addressee of Letter 39. Only two manuscripts - Codex Parisinus 1754 of the 12th c. and Codex Vaticanus 280 of the 15th c. ascribe Letter 40 to Sabinus. Moreover, Ambrose calls his addressee here "filius" whereas he usually addresses other bishops as "fratres". See Zelzer 1990: XXVIII.