The word 'NO' written in huge word lettering has been placed on the side of Ben Bulben.
Commuters in Sligo this morning said that the word had been written on the north side of the iconic peak.
The mountain is often associated with W.B.Yeats.
Speaking to Ocean FM from the mountain, Tommy Banks of Sligo for Life said that about 20 people were involved in putting up the sign.
He said the ‘N’ in the sign is 100m tall and that the sign is made of cladding. He said that the sign is temporary and will be taken down after the referendum.