You Do Need power plug adapter while traveling from Namibia to Northern Ireland.

Here is Why You Need An Adapter

The outlet and plug type used in Northern Ireland is G and in Namibia are D and M. This is how they look like.

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M

Northern Ireland

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

Type G