Irregular migration
United Nations and the International Organization for Migration have adopted irregular immigration and irregular migrant in preference to illegal immigration In some countries, breaching immigration laws is an administrative rather than criminal matter.
Undocumented migrants can mean both those who have arrived through irregular channels and those who have stayed longer than their visa had specified.
Migrant smuggling
" ... migrant smuggling is a crime that takes place only across borders. It consists in assisting migrants to enter or stay in a country illegally, for a financial or material gain. Smugglers make a profitable business out of migrants' need and/or desire to enter a country and the lack of legal documents to do so. International law requires governments to criminalize migrant smuggling, but not those who are smuggled. Since migrants give their consent to the smuggling venture, mostly due to the lack of regular ways to migrate, they are not considered victims in absolute terms. However, smuggled migrants are often put in dangerous situations by smugglers (such as a hazardous sea crossings), and might therefore become victims of other crimes during the smuggling process, including severe human rights violations."
UNODC https://www.unodc.org/e4j/en/secondary/human-trafficking-and-migrant-smuggling.html [accessed May 2024]
Subject searches to use in SOLO
Human smuggling is a subject search term. However, this topic may be included in works classified under broader subject headings such as: Emigration and immigration law or as related topic eg Human trafficking and Refugees -- Legal status, laws, etc
The International Law of Migrant Smuggling by 
Migrant Smuggling Irregular Migration from Asia and Africa to Europe by 
Migrant Smuggling by Sea by