Basically I am entitled to two as I understand it, UK and Irish.

I am coming up to renewal on my UK passport and not having done much looking am wondering if the Irish is cheaper what are the downsides of applying for an Irish passport? I guess the Post Office route isn’t an option? Anything else?

Are there any rules about passports and use of etc etc.

You can have two UK passports if you want (I have). One of our sales guys had 6 passports (from several different countries).

Irish is 80 Euros and Uk is about £80 IIRC. If you are getting kids the Irish are mush cheaper.

You can have two UK passports of you want (I have).

You can only have 2 if you have a legitimate reason, normally business and need supporting letter from your employer.

I am entitled to hold an Irish passport and considered getting one as you not top of the list to be beheaded by terrorists, unlike the UK and the US. However, it was quite a complicated procedure for me as my connection is through my Grandfather.

my connection is simple, as I understand it if a parent is Irish and it does say so on my birth certificate there isn’t much more to it than apply?????

You can hold both an Irish and a UK passport at the same time. People who travel a lot like to have 2 passports or dual nationality for travel to certain countries (like w_d implies) or to be able to pick the cheaper / better visa. If you don’t travel much, it makes very little difference.