Abstract. The Bronze Age stone alignments of S.W. Ireland were surveyed in detail to provide data with which to test hypotheses concerning their astronomical, metrological and morphological attributes. The astronomical tests are described in this paper. The hypothesis examined was that the centres of the stones of an alignment define a line which is orientated on an event of astronomical significance. The events considered as significant were the lunar standstill positions, the solstices and equinox. The alignments were treated as open-ended structures and their azimuths in both directions were considered. Two tests were carried out. In one, the allowable azimuth error was calculated individually for each site, and in the other, an average value was used. The probability level at which the hypothesis operates was calculated from a version of Bernoulli's Theorem and the results from both tests allow us to accept the hypothesis.