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

Here is Why You Need An Adapter

The outlet and plug type used in French Guiana are C, D and E 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

French Guiana

In French Guiana, outlets of type C, D and E are in use.

Type C

Type D

Type E