Brightside
Sorry to hear of your difficulties
First of all, don't panic.
As a retired person doing what you describe as ‘semi-casual’ work, it is highly unlikely that your father earned large sums through this work - especially, if as you imply, he was not the most organised man in the world.
If he was aged 65 years or over when he retired, it is quite possible that his tax liabilities are zero.
Where one spouse is aged 65+, a married couple is exempt from income tax if their joint income is below a specified threshold. This threshold was €34,000 in 2006. This threshold refers to the total of gross paye/pension income (eg per P60s), additional income (eg interest) and self-employment income after all allowable deductions.
As suggested above, it is essential that you get professional advice from an accountant or tax advisor in the first instance.
DO NOT under any circumstances approach Revenue without first getting this advice. If you give Revenue the impression that your father's income was a lot bigger than was actually the case, you could end up being badly ripped off in any subsequent settlement deal. Do not take for granted, for example, that all non-pension lodgements to his bank accounts are from self-employment income.
Don't be afraid to look for suggestions on how your father can legally avoid (not evade) tax. As a taxpayer your father has rights and entitlements just as much as anyone else and it would be a shame if these are not claimed by him or on his behalf.
A good accountant or tax advisor will be able to make this process as easy as possible for you and should have the experience and expertise to ensure that your father's and your family's rights are protected.