The Slieve Bloom Mountains are the oldest in Europe; sharing that title with the Massif Central in France, having formed some 400 million years ago. They were also once the highest at 3700m although they can no longer claim that distinction as the highest point today is 527m at Arderin.

Known as Shliabh Bladhma in Irish, they are named after the mythical Connacht warrior Bladhma who once sought refuge amongst the mountains.

The mountains as a whole form a link between the counties of Laois and Offaly, stretching from near Roscrea in the south west to Rosenallis in the north west. Established in 1985 to ensure conservation of the mountain blanket bog, the Nature Reserve itself covers 2300 hectares and runs along the north east, south west aligned ridge for approximately 25km.

The mountains consist of old red sandstone resting on silurian rock layers, surrounded by the flat limestone plain which dominates the midlands.