Killen has travelled widely to further his knowledge of the various mediums in which he works. To this end he has visited France ( with a particular liking for Brtittany ) being very much influenced by light in the area, Italy, U S A, Canada and the western parts of Ireland, especially Connemara.

A source of inspiration has been the work of the Irish impressionists of the late19 th / early 20 th centuries and at an earlier stage he was heavily influenced by the work of the Group of Seven in Canada. At present, he works in an impressionistic / expressionistic manner with the subject being painted influencing how the paint is applied.

His work is in private collections in Ireland ,Holland, Norway, England, Australia, U S A, Canada and in the collections of the Ulster Bank, IOM, Civil Service NI, Ulster Television plc, Boyle Civic Collection, MBNA, Haverty Trust and the RUA Diploma Collection. He was a prize winner in 2004 at the Discerning Eye Exhibition ,Mall Galleries, London.