The Airport and harbour police forces are tiny with limited ability to investigate crime. Also they have been around quite a while I think the Dun Laoghaire harbour police for example date back to 1836 making them the oldest uniformed force in the state. While the airport and port police forces have the power of arrest they then have to hand suspects over to the Gardai for questioning afaik.
BTW you mentioned a transport police needing, "commisoner, detectives, sergeants and inspectors", with the exception of a commissioner, all those others would be needed anyway even if they were a separate division within the Gardai.
The only real extra cost would maybe be IT and HR and if you wanted you could share those with the Gardai, while keeping everything else separate. Many county councils do that throughout the country.
I think people are greatly overestimating how much it would cost as a separate division.
Even if it was a division within the Gardai, you'd still want them to hire extra Gardai/Transport Police rather then taking from other divisions. You'd still want to open a new police station at/nearby Hueston. You still want to train them up in the specifics of working around rail. You'd still need detectives, sergeants and inspectors. All those costs would still be the same either way.
The only question would it be more effective if it was independent or not.