easier to use an example or two if you have some... like manual searches you need a certain level of clues, otherwise there's no way to confirm if you have a possible match. As well as the name, dob and county, you ideally need one or more of these :
parish or town
parents names..ideally both but one can be enough sometimes
siblings names and dates
The names make a big difference - common names you need more details, rare names you can sometimes get away with just a name, year and county. Some name variations and Latin versions are allowed for in the searches but you can also just enter the first few letters - e.g. Ja
for James/Jacobus etc.
Baptism searches are much easier for RC families since they include mother's maiden name, CofI baptism generally dont. However the search facility on RootsIreland doesn't always work correctly for maiden name searches - think that might relate to certain counties or parishes...
For the actual baptism searches go to the relevant county section, and enter the basic details - and see how many matches there are. Fine tune the search by entering the first names of the father and/or mother if you have them and again check the number of matches. If you get a small number of results, then you can try each of the parishes in turn and note the number of matches in each. Once you have narrowed down a parish you can then search within that for siblings by leaving the parents names and trying first letter of all names in turn - starting with the more common ones P, M, C, K, W, J etc (they dont allow a general search by parish without a name).
Basically the aim of this is to get to a search that gives you a small number of results, and ideally the message '1 match for the search criteria...' Once you have done this you can follow up by purchasing the record transcript(s), or better still viewing the film in the NLI. Obviously if you get a 'No records were returned', or a very a large number you are in trouble...and probably have to go back to your clues and see if you can add to these.
If you dont know the county then the process is more complex, but if you believe the person came from one of the areas covered by RootsIreland then you can try the "full" search with parents names and see if there are results - if so then de-select the various counties in turn until you narrow down where they are - then select the relevant county centre(s), and start the parish search.
Re: Duplicates - some
of these relate to baptism and civil births for the same person.
Re : Searching within county - you can search by parish, but an initial is required