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

Here is Why You Need An Adapter

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

Northern Ireland

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

Type G

Swaziland

In Swaziland, outlets of type M are in use.

Type M