You Do Need power plug adapter while traveling from São Tomé and Príncipe to Northern Ireland.

Here is Why You Need An Adapter

The outlet and plug type used in Northern Ireland is G and in São Tomé and Príncipe are C and F. This is how they look like.

São Tomé and Príncipe

In São Tomé and Príncipe, outlets of type C and F are in use.

Type C

Type F

Northern Ireland

In Northern Ireland, outlets of type G are in use.

Type G