There’s no difference between the information on the irishgenealogy version and a paid for copy. You can order a research copy (ie a photocopy) for 4 euros or a certified copy (such as you would need for legal documents) for 20 euros, but it’s normally the same image you see on the screen. (Occasionally they type them if the handwriting is very faded or very hard to read).
Irishgenealogy are gradually putting the records on-line free. 1878 happens to be as far back as they have got with deaths so far. 1878 to 1890 were only added a few weeks back so your timing is good. The remaining deaths back to 1864 (which is when they started death registration) are to be added next year.
Be aware that the irishgenealogy site only has marriages and death records for what is now Northern Ireland up to 31.12.1921. Births stop in December 1917. If you need records after that you need to use the GRONI site, save that births within 100 years, marriages 75 and deaths 50 are not displayed on the on-line site. There are ways of getting them though.
Irish birth, death & marriage records in the 1800s are the same format as English ones. There’s less information than you get on Scottish certificates. No mothers name on a marriage certificate and parents names are not routinely recorded on death certificates, save perhaps for young children.
