you can order certs by post from the GRO using the index details from the familysearch site - their website is
www.groireland.iethe details they requires are name, record type (e.g. marriage, birth etc), registration district, year/quarter, volume and page. The other details are LDS/familysearch references and are not required to order a cert.
My Cantwell line is a bit of a brick wall at the moment - a gg-grandmother is Sarah Cantwell born c1830 in Co. Wicklow.. but I cant find her birth, so I have not been able to go any further. The closest I have found so far is a Cantwell family owning land quite near to my Fitzsimon family - but have found no definite links yet. Strangely there are absolutely no Cantwell's listed on Co. Wicklow on the 1911 census..
Shane