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!