The Writings of Irenaeus are available on-line at the Ante-Nicean Fathers website.
I've searched all through his writings and can't see where he ever mentions "Lord's Day" or the "first day".
I do know that there is a quote that has been falsely attributed to him but actually doesn't appear in his writings.
Please indicate the quotation you are referring to and where it appears in his writings.
Thanks!
Then we all rise together and pray, and, as we before said, when our prayer is ended, bread and wine and water are brought, and the president in like manner offers prayers and thanksgivings, according to his ability,147 and the people assent, saying Amen; and there is a distribution to each, and a participation of that over which thanks have been given,148 and to those who are absent a portion is sent by the deacons. And they who are well to do, and willing, give what each thinks fit; and what is collected is deposited with the president, who succours the orphans and widows and those who, through sickness or any other cause, are in want, and those who are in bonds and the strangers sojourning among us, and in a word takes care of all who are in need. But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Saviour on the same day rose from the dead. For He was crucified on the day before that of Saturn (Saturday); and on the day after that of Saturn, which is the day of the Sun, having appeared to His apostles and disciples, He taught them these things, which we have submitted to you also for your consideration.