I would get a carpenter to come out and look at it first. It could be salvageable with the addition of slips of wood top and bottom. If you go with this option try to get the door dipped first so that any patching and addittions fit in seemlessly once the door is repainted.
If replacing, then you can do the same fix with a new off the shelf door to make it fit. A brand new door can be made to the sloped shape, but no joiner would recommend that. Sky is the limit in price for a new door, but 1500 should do it. Crossan's Joinery in Phibsboro are very good.
I would be more worried about the door frame being crooked as no matter what you do with the door it will always look off cos of the frame being off. It might be an option to take out the frame aswell and refit. We replaced a frame and took all the scrolls & carvings off the old one, got them dipped and then remade the new frame to a similiar design, with the scrolls. It didn't look the exact same but was close. The scrolls & detail below the fan light were really the only important bit. We took pictures before ripping it out. Also needed to wait a while before repainting to ensure the chemicals in the dipped wood were gone.(new paint would have flaked otherwise). There is a dipping place just before the canal bridge to Harolds Cross. One of those old antique shops. Can't rem their name but they did all our dipping and were great. Expect to pay e30 approx per door.
Best off seeking a carpenters advice in all this as there may be more to it than just replacing or fixing the door.