Unless you do many hundreds of hours a year (unlikely for private flying) by far your largest costs will be fixed, irrespective of the amount of flying you do. You need to work those out first and then make a guess at how much flying you will do - it will in practice turn out to be less than you expect/hope now. Then you can factor in fuel and maintenance costs.
Or you could do what most people do. That is they just do it and live in penury for ever after.
Or I could say "if you have to ask you cant afford it". But that would be unfair without pointing out that there are much cheaper options in terms of aircraft ownership than the Cessna/Piper route you are proposing to go down - in the UK at least (permit aircraft) but I'm not sure how it works in Eire.