Declan grew up on the Northern Beaches as an avid surfer and is a member of Freshwater SLSC. His swimming talent was identified at an early age, with his first major success coming in the 2024 Australian Age Championships, where he won 7 gold medals across all strokes in the 16-18 years bracket. 

At the 2024 Australian Short Course Championships, Declan broke the world record in the men’s 50m Butterfly (S14) which led to him being the youngest male swimmer selected to compete at the 2024 Para Swimming European Open Championships. 

In September 2025, Declan competed at the World Para Swimming Championships in Singapore, reaching the final in the Men’s 100m Backstroke (S14) placing 6^th, Men’s 100m Butterfly (S14) placing 8^th and Men’s 200m Freestyle (S14) placing 6^th.