Proof (Demonstration) of the Apostolic Preaching, §54 — J. Armitage Robinson, St Irenaeus: The Demonstration of the Apostolic Preaching (SPCK, 1920) (public domain)
54. For this cause then is He Saviour. Now Emmanuel is, being interpreted, With you God; or as a yearning cry uttered by the prophet, such as this: With us shall be God; according to which it is the explanation and manifestation of the good tidings proclaimed. For Behold, He saith, the virgin shall conceive and shall bring forth a son; and He, being God, is to be with us. And, as if altogether astonished at these things, he proclaims in regard to these future events that With us shall be God. And yet again concerning His birth the same prophet says in another place: Before she that travailed gave birth, and before the pains of travail came on, she escaped and was delivered of a man-child. Thus he showed that His birth from the virgin was unforeseen and unexpected. And again the same prophet says: Unto us a son is born, and unto us a child is given; and his name is called Wonderful Counsellor, Mighty God.
Reproduced in full. Translation: J. Armitage Robinson, St Irenaeus: The Demonstration of the Apostolic Preaching (SPCK, 1920) (Public domain).
Why Pitre cites it
Irenaeus reads Isaiah 66:7, 'before she who was in labour gave birth,' as the Virgin's unexpected and extraordinary birth of Christ — the earliest witness Pitre has for a painless birth at Bethlehem.
Notes
Robinson's wording of Isa 66:7 is 'Before she that travailed gave birth, and before the pains of travail came on, she escaped and was delivered of a man-child' (not 'brought forth'); 'the virgin' is lower-case in Robinson ('His birth from the virgin was unforeseen and unexpected'). Footnote numbers and page markers removed; an OCR split 'H e' corrected to 'He'. Not on CCEL/New Advent; tertullian.org hosts the full 1920 text.