Relative clauses in Scottish Gaelic act as nominal modifiers such that they modify a noun and not a phrase. They always occur post-nominally and are headed by the relativiser a.
Relativization of the object of the preposition:
An duine a bha thu ag èisteachd ris. the man Wh-C be.PST 2SG PROG listen.VN to.3SG.M 'The man you were listening to'
Preposition fronting version of the above sentence (acceptable to Lewis and Uist speakers, variable for Skye speakers):
An duine ris an robh thu ag èisteachd. the man to.3SG.M Wh-C be.PST.DEP 2SG PROG listen.VN 'The man you were listening to'