Snack foods tend to be highly emphasized in McDonagh's plays, touched on in this article: "The Lord-Lieutenant’s Biscuit and the Irishman’s Spud: Ireland’s Iconic Snacks Blasphemed in Martin McDonagh’s Leenane Trilogy" By Christopher Morrison.
But McDonagh's use of snacks in a dramatic context is not restricted to the Leenane Trilogy--they are something of an obsession in The Cripple of Inishmaan, and they are often a point of passionate, sometimes violent, contention.
The linked article goes into great detail on the historical and sociopolitical implications of McDonagh's use of snacks, but that type of thematic foundation alone is not sufficient to carry a dramatic work. McDonagh's plays are celebrated and highly popular outside of Ireland, so there must also be a more immediate, relatable, and universal purpose for the device.