The Transit Circle is situated in the single storey east wing. It is a superb example of Grubb's workmanship and is undoubtedly the most complete and best preserved instrument of its type in Ireland today. Transit instruments were employed to measure accurate positions of stars; the basic raison d'etre of a 19th century observatory. The Transit Circle was designed with a view to mechanical stability - an important requirement for such accurate work. This, together with the difficulty astronomers had with reading metal scales by reflected light, lead Grubb to implement its most original feature, namely the use of 30" declination scales inscribed on glass and read with transmitted light. This was a new development by Grubb which has subsequently found favour by many astronomical and surveying instrument manufacturers. (John Butler)

The Transit Telescope in the Crawford observatory (with a 5" diameter lens) is likely to be the only one made by Grubb.

It is currently being restored by Mr Bertie McClure.