Thanks for clarifying. Clears a lot of issues up. I think you're in the right, legally, but like Clubman said enforcing it will be difficult and probably expensive and stressful.
I'm being pedantic but its only 1.4km .8 of a mile from Heuston to Killmanham/SCR. About a 20 min walk. 10 min cycle if you had a foldup bike in the boot. Anyone I know with arthritis was encouraged to do light exercise. I accept, that maybe your case is different. Theres a decent hill up to Killmanham which you'd feel with arthritis. But most people should be doing 30~60mins of walking a day.
I don't know that area well but I don't remember any car parks in that area alright. Maybe theres one on the bus route though thats a compromise. Maybe drive around the area looking for companies that have spaces that don't look busy and see if any would rent you one. Even apartment blocks etc.