The results page gives you lots of information, including name variants for the parish and clear details of what is actually available by record type and time period. I particularly like the image count so you can judge whether you want to browse the whole register or make use of the filter facility that should take you directly to the start of the year you are interested in. Having said that, I didn't find that this feature was working too well with the registers I chose for some reason but it was no hardship to scroll back through the pages and anyway, paper was expensive in days gone by and priests would use every scrap wisely, so finding records out of step when you are browsing is not uncommon. This may not have all been the priests fault though - I've been told that pages may have got out of order on the microfilm, so I'm afraid that there may be nothing for it but to do it the old-fashioned way by starting at the beginning and reading through. That way you will also you get to know the families in the community and start to see sibling births and inter-marriages between families that you will never find through the limited search screens on Roots Ireland (IFHF).
The images might take a moment or too to resolve on screen - be patient - and several are poor quality, but there are brightness controls to play around with to see if things become clearer. Remember too, that you may encounter many records in Latin, but remember the context of what you are browsing and common abbreviations will start to make sense e.g. "Conj." for "Conjugatus" or "Married" in front of marriage entries.
The records are far from complete but its easy to compare what the NLI are making available versus what is thought to exist elsewhere outside local custody by checking the NLI results page against the entry for the parish returned from the Irish Ancestors RC parish maps. Do bear in mind though that the majority if not all of the other sources of RC records that might be quoted on the Irish Ancestors site are likely to be the transcripts done by county family history societies that have been published via Roots Ireland and you will probably need to take out a subscription to search them.