Irregular migration refers to movement that takes place outside the regulatory norms of the sending, transit and receiving country. The irregularity does not refer to the individuals but to their migratory status at a particular point in time. People may have irregular migration status for a variety of reasons and in many cases people will be able to regularise their status with the help of immigration advisers or solicitors.
When working with people with irregular status, you need to be mindful of the Immigration and Asylum Act 1999 and OISC regulation. Unless you and the organisation you work for are appropriately qualified and regulated to do so, you must not provide any immigration advice or services. You can however provide general information on the immigration and asylum system in the UK, signpost to websites and advice lines where clients can find information about immigration issues and refer people on to immigration advisers or solicitors. Information on finding an immigration adviser can be found on https://www.gov.uk/find-an-immigration-adviser.
Homeless Link has produced a Migrant Destitution Toolkit that provides useful resource for both frontline workers and senior managers who work in services that provide either regular or occasional support to destitute migrants from outside of Europe. It can be found on http://www.homeless.org.uk/our-work/national-projects/strategic-alliance-on-migrant-destitution/migrant-destitution-toolkit