If the French suggestion works for Calais, why not for Ireland?

You mean apart from the fact a port is completely different in scale to a 499km border, that all the French system achieves is to separate the traffic into red and green channels where the red have to be checked, that building such infrastructure as the channels is expressly prohibited by the withdrawal agreement and that the police of Ireland have expressed serious security concerns about any attempt to have such infrastructure? Give me a minute and I will think of something.