Are there any restrictions on the acceptance into the Portuguese International Ship Registry?

Age:

There are no direct restrictions on the age of the vessels that may be registered, but for ships exceeding 15 years an additional pre-survey might be applicable depending on the ships' PSC and Classification records. 

Surveys:

Vessels to be registered may be subject to a pre-survey based on the ships' records, age and detention history.

All vessels registered in MAR must have their relevant statutory certificates issued by one of the classification societies which have been authorized as a Portuguese Recognized Organization (RO). These entities may also perform surveys and inspections on behalf of MAR, as well as the undertaking of certain tasks. Currently, Portugal recognizes eight classification societies in order to undertake some of its tasks within the framework of MAR:

Other classification societies (IACS members) may be recognized by Portugal in the near future.