Gallarus Oratory is a rather mysterious dry-stone building on the Dingle peninsula, which is perhaps a thousand years old. It has been variously identified as a Christian church, a shelter for pilgrims, and a private funerary chapel. No-one really seems to know, and initial cuttings have not revealed definitive information about either period of construction, or reason for use. It is amazingly well-constructed, with the stones being very carefully lined up to produce the solid wall effect you see.
The Luas light rail bridge in Dundrum, Dublin, named after William Dargan, a famous railway engineer.