It's great that you're interested in climbing these two special mountains! :) Are you thinking of visiting us in the near future?
On the official Irish Trails
website you can find a description, helpful information and reviews from other hikers for both the famous Croagh Patrick
Heritage Trail and Diamond Hill
. The biggest difference in these trails is mainly the length, with Diamond Hill taking about 2 hours, and Croagh Patrick taking 3.5 hours
Is there a certain type of trail or trek that you're looking for? You can find more information about walking and hiking on the island of Ireland, as well as other available trails here
Take a look at this Tell Your Story thread
from Elly too, as she tells about her experiences climbing up Croagh Patrick. The thread is in Dutch, but you can use the translation option on the Community to read the text in English. :)
I hope this was helpful, and that you'll receive more tips, experiences and suggestions from other members.