It carries a bit of a playful, endearing tone, implying a kind of affection and familiarity with the older generation.
In the context of *Translations*, the phrase "aul fellas" is used to refer to the older men of the village, who are resistant to the changes brought about by the English surveyors who are imposing new English names onto the Irish landscape.
The term highlights the generational divide and the conflict between tradition and modernity that is at the heart of the play.