I applied for Irish citizenship, and subsequently a passport, in 2017. My grandfather was born in Dublin, and I was very pleasantly surprised when I discovered that was enough. I had to do a bit of preparation, which involved getting a copy of his birth certificate from the Irish authorities, and his marriage and death certificates from the UK (he lived his entire adult life here). Then I had to supply my mum's birth certificate and marriage certificate and my birth certificate to prove my ancestry. That got me on the Foreign Births Register as an Irish citizen. Cost me about 300 quid and a couple of months to sort out, all told. You need a solicitor to witness the signing of various forms, but I had a suitably qualified work colleague at the time who was happy to do it for me, so I didn't need to pay for that.
I now have both Irish and UK passports, but as Irish citizens have unlimited rights to live and work in the UK, I'll probably let my UK passport lapse, as (a) it now gives me far fewer rights and (b) I'm not buying into the hype about them being blue. The only thing I I currently use my UK passport for is ID, but pretty much everywhere is happy to accept the Irish one for that as well.
First time I travelled on my Irish passport was to Ukraine in 2019. It felt rather weird, but I'll get used to it in future.