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

Here is Why You Need An Adapter

The outlet and plug type used in San Marino are C, F and L 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

San Marino

In San Marino, outlets of type C, F and L are in use.

Type C

Type F

Type L