The Lahart Garages you refer to are owned by one of my father's brothers. However while the name might coincidentally sound a bit French it is not French. The Lahert\Lahart\Laherty etc derives from Ó Flaithbheartaigh (Modern Irish 'Ó Flatharta, anglicised as 'O'Flaherty').
The spelling has changed and alternated over generations.
Paul Lahert email:
paul@riteweigh.ie Dublin,
Ireland.