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Agglomerate or Vent Agglomerate is a volcanic breccia consisting of angular volcanic fragments set in an ash-like matrix. Some agglomerates contain ‘volcanic bombs’.

Local agglomerate was formed during explosive degassing of vents associated with Tertiary Volcanic Activity of the Slieve Gullion Complex 60 Million Years ago. Geologically agglomerates outcrop with felsites and granophyres in this area and is rarely used as a building stone. However, as with all local stone it was used in the past and is seen as random rubble and quoins stones on ancient monuments in this area.