Usually service is by registered post. If registered letter refused Plaintiff may apply to the District Court Judge for an order for substituted service. Evidence is by affidavit showing the returned envelope, the report from the post office, and evidence that the proposed defendant still lives there.
Such evidence could be e.g. the result of a personal call, extract from the Electoral Register etc
Letters sent by normal post with a certificate of posting at various times including the day before summons. Letters have not been returned.
Asking neighbours if person still resides at location. However, it will probably not be possible to receive this verification in writing. Is acheiving it verbally acceptable in court?
Letters sent by normal post are not proof of receipt, only of posting and would not on their own be sufficient as if the property empty they would simply be left in the property.
It would probably be best to engage a PI company to provide a report stating that the other party lives there whcih could be produced to the court. A court may not be satisfied if you, as the plaintiff, are also happy to swear that the defendant resides at the address you provide without independent evidence.