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

Here is Why You Need An Adapter

The outlet and plug type used in Northern Ireland is G and in Pitcairn Islands is I. This is how they look like.

Pitcairn Islands

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

Type I

Northern Ireland

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

Type G