Cameron Landscapes, just outside Belfast, was praised for a private garden with pond, clipped hedging and herbaceous borders, said managing director Geoff Cameron.
"The awards mark the end of another difficult year for the landscape industry," said a spokesman for the ALCI. "Public spending cuts and the downturn in the property market have taken their toll."
Portadown-based sports-ground specialist Clive Richardson was also praised for building a pitch at the Aviva Stadium in Dublin, home to the national rugby and soccer squads.
ALCI represents firms involved in commercial and private landscaping and maintenance across Ireland, and the awards alternate annually between the Irish Republic and Northern Ireland.