Chapter 46 has McGonnagal think of Trelawny as the origin of the prophecy, and she seems to remember it in its original voice.
The sequence of events then seems to be that Trelawny went into her trance with McGonnagal in the room, but the destined recipient of the prophecy actually being Snape just outside it. Snape thought he was overhearing a prophecy meant for McGonnagal, when in reality McGonnagal overheard a prophecy meant for Snape.
Edited to add: Chapter 28 is the one that gives the most detail.
Chapter 46 has McGonnagal think of Trelawny as the origin of the prophecy, and she seems to remember it in its original voice.
The sequence of events then seems to be that Trelawny went into her trance with McGonnagal in the room, but the destined recipient of the prophecy actually being Snape just outside it. Snape thought he was overhearing a prophecy meant for McGonnagal, when in reality McGonnagal overheard a prophecy meant for Snape.
Edited to add: Chapter 28 is the one that gives the most detail.