You Do Need power plug adapter while traveling from Cook Islands to Ireland (Eire).

Here is Why You Need An Adapter

The outlet and plug type used in Ireland (Eire) is G and in Cook Islands is I. This is how they look like.

Cook Islands

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

Type I

Ireland (Eire)

In Ireland (Eire), outlets of type G are in use.

Type G