McLachlan was born in Dublin, son of the writer Leland Bardwell, and studied at the DIT Conservatory of Music and Drama (1982–6), the Royal Irish Academy of Music (1989–97), and Trinity College Dublin (BA 1988), where he received a Ph.D. in musicology in 1999 for a study of the relationship between analysis and compositional technique in the post-war avant-garde.
He has also represented Ireland at international festivals, including the ISCM World Music Days in Slovenia in 2003 Croatia in 2005 and New Zealand in 2022.
In 2006, his work Grand Action was commissioned as a test piece for the AXA Dublin International Piano Competition.
A critic wrote of a recording of McLachlan's piano piece Nine: "The style of each little piece sends one's imagination and musical memory reeling, some of them evoking French Impressionism, some jazzy in feel, some reminiscent of the miniatures for piano of Webern, and none of them in any way, shape or form derivative.
The material within these sections is propelled by a rigorous focus on subtle rhythmic and melodic permutations, which result in both surface opacity and gradually increasing tension.