If you do a lot of nixers and don't report your earnings for tax purposes, you may be accruing a tax liability.
Your mortgage application form may specify that you list "all sources of income" for the purposes of assessment.
If it does and you don't and you get a mortgage and this matter is later exposed, you could be in difficulty with your mortgage.
In reality if you are appoved your mortgage on the income you declare and you can continue to make repayments there is no reason that you would be on the banks or anybody elses radar.
The tax liability is something I imagine you're well aware of, if they are nixers and that is an issue!
Regularise your tax situation as there is a danger that you may be hit with interest and penalties which will reduce your ability to pay your mortgage.
Revenue are very good at figuring out if people are living beyond their means - if you have a house that you obviously cannot afford to pay for out of your taxable income, it will raise alarm bells in Revenue and likely prompt investigation.