Motorcycle Clothing. HELMETS ARE COMPULSORY THROUGHOUT EUROPE.
A full-face or flip-front helmet is preferable as wind/insects etc. can be a problem on the motorways with other types. It does rain sometimes and a visor is recommended. Some helmets that are legal in other countries may not meet the Irish or UK requirements, (I'm thinking of some helmets available in the USA that I have seen), so please check beforehand if you are hiring a motorcycle and bringing your own gear. The hire company will be pleased to advise you, and most of them can supply a suitable helmet if required for a small sum.
Apart from that, anything goes... We DO encourage you to wear proper protective motorcycle clothing. Jacket, trousers, boots and gloves with appropriate armour in the necessary places.
Normal Off-the-Bike Clothing.
Most people bring far too much the first time they go touring. The sort of places I favour to stay in are fine for casual clothes so jeans, polo shirt, jumpers etc. are ok. Look for lightweight trousers as they take up a whole lot less space, likewise shoes... get a comfy pair that can be packed reasonably flat, trainers are good.
During the day you will probably be wearing your biking gear so you will probably get away with a couple of changes of clothes, certainly for a shorter tour anyway. T shirts can be easily washed if required, as can underwear and socks.
For the continental tours I bring shorts and sandals, and also swimming costume as some of the hotels we use have swimming pools or are beside lakes with bathing facilities.