To be fair to the Nigerians a lot of them tend to work for their money.... often taking jobs very few folk want such as driving taxis, buses, luaseyes, cleaning, security, etc...
As DM notes, a lot of them drive taxis and, obviously, a lot more do security. These sectors are traditionally very poor contributors to the exchequer so it seems fair to question whether they are paying appropriate tax on the money they send home. As Leo noted, us Irish sent home large fortunes in used notes over the years and I think we can be pretty sure a lot of that was from into the hand work and/or other corrupt activities such as welfare fraud. However, these days there is a lot more traceability, cash in an envelope is unlikely to be the first choice.
Outfits like Western Union, Banks, etc ought to monitored closely... I know there are rules for reporting cash transactions over €10,000 but what about those transactions that are slightly under the limit or multiple transactions, disguised or otherwise, which collectively exceed the threshold?
The so-called gig economy ought to come under more scrutiny. The faceless bureaucrats in Brussels ought to make our Govt introduce withholding tax for firms like Airbnb, Free Now, Uber, etc... I have been told, for example, that Free Now's top driver had takings of €133,000 last year in respect of base jobs alone. This is serious money, not just a few nixers that can be reasonably overlooked. Similarly, a withholding tax regime within the security trade would undoubtedly benefit the exchequer. I know a good few soldiers who done door work over the years on a cash in hand basis, in fact there was a general acceptance that it wouldn't be worth their while giving up their weekends if they had to pay income tax on the relevant remuneration.
However, it's important not to get carried away and tar all immigrants with the same brush. Don't forget many of those who don't hold PSV or PSA licences are working for less than minimum wage*.
*source: DM.