In Ireland there's no 'rules', however people do tend to do similar.
There is rarely a gift list/register , people did give gifts many, many years ago, all the usual things for the house etc however as most are living together a while when they marry they usually have everything they need.
Cash is always welcomed, the Irish don't get offended at receiving cash gifts, most prefer cash. They can use it towards things they need and the honeymoon etc
If someone is going away to a Country with a different currency people sometimes give that currency as a gift .
There's no set amounts to give, but usually for a couple attending a friend's wedding they will give between 200 -300 euro per couple, for immediate family anything from 400 euro per couple, of course they can give more or less if they want.
It's always said to cover your meal if unsure so it depends on the venue. I've been to many weddings over the years, there are couples who really don't care and just want you to celebrate with them and there's the ones who'll bitch to everyone about your stingy present. The invitation usually lets you know , if they say cash only presents get saving 🤣 if they say no gifts, they mean they do want something small.