Brían, thanks for the Treoir reference; I'll try to track it down, and let you know what I find. I ought to have said at the outset that the song is partially translated in the notes to the CD of Róise's songs, and that the English translation given for the last two verses (which are all that was recorded, or maybe all that survived of the original recording) is as follows:

As he was going through Ranafast, his ribs were broken,
Bacach Síol Andaí, Bacach Síol Andaí,
As he was going through Ranafast, his ribs were broken,
Bacach Síol Andaí, Bacach Síol Andaí.

As he went west to Cloghcor, they set off in pursuit of
Bacach Síol Andaí, Bacach Síol Andaí,
As he went west to Illion, the litany was recited for
Bacach Síol Andaí, Bacach Síol Andaí.

As for Pakenham's account its not so much that I doubt his veracity, but that as he himself admitted in his preface the documentary evidence he used was very heavily drawn from the records on the British side, so that I find it hard to accept some of the statements he makes as being as impartial as he would have us believe.