According to dictionary entries I can find, you're right. For example, "The Gaelic-English Dictionary: A Dictionary of Scottish Gaelic" by Colin B. D. Mark says,
b'àill leibh? (for dè b'àill leibh?) pardon? / excuse me (I didn't hear)
But the audio suggests that it's pronounced with a short a, which would not require an accent. To add to the confusion, the episode of Speaking our Language dealing with Coping with language difficulties lists b' àill leibh? in the vocabulary, but in the conversation transcript gives b' aill leibh, which is what it sounds like to me when watching the clip from the programme. Some clarification would be useful.