1) They speak Gaelic
I know, you're probably like 'DUH!' but I don't think this was something that I'd ever really come across. On arrival, I suddenly saw all these signs that were not in English and was suddenly like 'Oh riiiiight'. It makes sense obviously but wasn't anything I'd ever come across before. You don't need to try and learn the language though - everyone speaks English and pretty much everything is in English. It's just that their original language has been preserved as well.
2) You don't have to go through passport control if you're coming from the UK
It really freaked me out when I flew from Dublin back to London as I was ushered through a side door that allowed me to skip passport control. I was "wait.... what?" It really freaked me out and I wasn't quite sure why I didn't need to go through any checks. Turns out, there's a Common Travel Area between Ireland and the UK, which means passport control isn't required.
3) It's more like the UK than I thought
I always think of Ireland as being more strongly associated with Europe than the UK. However, I shouldn't have been surprised to discover that they have UK power sockets and drive on the left as they are physically closer to the UK. Yes, leave your continental power adaptors at home as they use have UK sockets.
4) Music is everywhere
Once you start thinking about how many famous Irish musicians and bands there have been (The Pogues, The Cranberries, Sinead O'Connor, My Bloody Valentine, Thin Lizzy, Ash, Two Door Cinema Club - oh yeah and U2, The Corrs, Hozier, Snow Patrol and Boyzone) then you start to realise how much music comes from out of the island. There are so many pubs where you can go and watch locals sitting around playing traditional music for hours on end, and you should definitely do this at least one evening on your trip. I recommend the proper old boozer The Cobblestone, where the drinks are cheap and the locals are friendly.
5) It's not that big
Ok, so I was surprised by how small Dublin is. You can walk from one end of the city to the other in under an hour. The city has only about 553k people, while Greater Dublin is just shy of 2M. The whole of Ireland is only 4.7M, which is less than the population of Sydney (5.1M) - a fact that blows my mind. I think it's because so many Irish people so many people have left the country for numerous centuries that our sense of people who are Irish are spread far across the globe (apparently, 80 million people worldwide claim to be of Irish descent). So I was definitely expecting Dublin to be a much bigger city but in fact, I think its size made it very charming. It felt very charming and cosmopolitan, packing a lot of punch into its small size.