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

Here is Why You Need An Adapter

The outlet and plug type used in Denmark are C, E, F and K 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

Denmark

In Denmark, outlets of type C, E, F and K are in use.

Type C

Type E

Type F

Type K