SW News:

Seeking the renewal of the Catholic faith in Ireland, Holy Masses were offered at Mass rocks in all 26 dioceses of the country. Masses were said on rocks that served as altars centuries ago when priests secretly offered the Holy Sacrifice in secluded locations at a time when the Catholic faith was outlawed. 

The initiative was started by the charity Aid to the Church in Need seeking the renewal of the faith through the intercession of the Irish Martyrs. 
Following the Protestant Reformation, the English rulers persecuted the Catholic Church in Ireland. Lay people were penalized with fines and imprisonment if they were found practising Catholicism. 

Priests were also severely punished if they were found administering the sacraments. Many of them were even put to death. 

Some of the rock altars are located along the coast while others are on mountaintops. Some stone altars are located in grassy fields or under pine trees. The outdoor altars can be seen from County  Donegal in the north of Ireland to County Cork, in the south.