Legally all dogs over the age of 12 weeks must be microchipped. All dogs advertised for sale also need to be microchipped and registered with one of the four approved Irish databases, Animark, Fido, IKC or Irish Coursing Club. Fines can be imposed for being in posession of a dog that is not microchipped.
You can click to see some public details of the dog microchip on the Europetnet website. Always ask to see the microchip cert when you view a dog and crosscheck the owners details.
Laws about microchipping are covered in the Microchippng of Dogs Regulations 2015 (S.I. No. 63 of 2015), section 3(2).