"We wrote an interesting book together."
Translation:Scríobhamar leabhar spéisiúil le chéile.
I think it's because of the consonant cluster scr- at the beginning of the word. /S/ won't lenite if it's followed by certain consonants, such as /p/, /t/, /m/, etc, because it would supposedly be too hard to pronounce (as if that's ever stopped the Irish). And yet, the past tense of snámh is shnámh...