On August 2nd 1880, the official time in Ireland was set to be Dublin Mean Time. This was probably administered from Dunsink Observatory to the northwest of Dublin city.
The offset between Dublin Mean Time and GMT would have largely been determined by the Longitude of the Observatory. I measured the location of the Observatory on 22/12/2002 with a hand-held GPS unit. There are two domes, one on the main building and a smaller one. Below are the measurements I took near each with a photo of where I was standing relative to the building.
Latitude Longitude Elevation Offset Near 53.38741 -6.33839 80m 0h25m21.21s main 53.38756 -6.33856 73m 0h25m21.25s main 53.38722 -6.33842 79m 0h25m21.22s main 53.38712 -6.33769 80m 0h25m21.05s small 53.38701 -6.33746 81m 0h25m20.99s small 53.38718 -6.33738 79m 0h25m20.97s small
Ian Elliott made some additional measurements with a camera GPS in 2013, and found the Meridian room to be at Meridian Room to be 53d 23' 13''.5 and 6d 20' 20''. Measurements for the South Dome game 53d 23' 13.5'' and 6d 20' 15''. In mid 2015, Tom Ray verified the coordinates of the telescope in the South Dome as 53d23' 13''.5 and 6d20'15''2, and height 75m.
All these would correspond to an offset of 25 minutes 21 seconds, to the nearest second. This agrees with the comments in the Unix zoneinfo file:
# Whitman says Dublin Mean Time was -0:25:21, which is more precise than Shanks. # Perhaps this was Dunsink Observatory Time, as Dunsink Observatory # (8 km NW of Dublin's center) seemingly was to Dublin as Greenwich was # to London. For example: # # "Timeball on the ballast office is down. Dunsink time." # -- James Joyce, Ulysses
Though I haven't checked Whitman or Shanks myself.
Note from JC: I would expect that the time standard was set from the transit circle instruments in the Meridian Room at the back of the observatory (about 20-30metres west of the first measurement position). That is where the dials of the clocks were originally installed to allow readings to be taken at the telescopes.
Patrick A. Wayman's book "Dunsink Observatory 1785-1985" gives some historical latitude and longitude measurements for Dunsink. The figures quoted for longitude are:
The 1935 Nautical Almanac (p. 684) uses these figures, which show that the altitude is pretty good too.
No Place Long Lat Alt 80 Dublin, Irish Free State +0h25'21.1'' 53o23'13.1'' 86m Dunsink Observatory, Trinity College
Interestingly, the values reported in the Astronomical Almanac from 1992 onwards seem to be incorrect, swapping the final digits of the latitude and longitude. This was pointed out to me by Ian Elliott in September 2013.
One of the other important observatories in Ireland is Armagh Observatory. A trip there with a hand-held GPS gives a location of around 54o21'10.2''N, 6o38'52.6''W, 60m. This seems to be in reasonable agreement with standard figures of around 54o21'12'' and 6o38'54.