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

Here is Why You Need An Adapter

The outlet and plug type used in Pitcairn Islands is I 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

Pitcairn Islands

In Pitcairn Islands, outlets of type I are in use.

Type I