The Botanic Gardens has an excellent collection of oak species, both evergreen and deciduous. The sessile oak is one of the two oaks native to Ireland (the other is Quercus robur the pedunculate oak). The two species can hybridise so identification can be difficult. The oaks at the south end of the main lawn are Q. robur, having long stalks on the acorns and leaves with a very short leaf stalk. This tree, in the corner of the great lawn nearest Botanic Avenue is Quercus petraea. It has long petioles (leaf stalks) and short peduncles (acorn stalks). The leaves of Q. petraea lack the little ‘ears’ (auricles) at the base of the leaf that are a characteristic of Q. robur. See comparative photo below. 

For information about Irish oaks see Flora Hibernica by JR Pilcher and VA Hall (Collins Press)

Photos taken in Belfast Botanic Gardens in 2007. Copyright: Friends of Belfast Botanic Gardens.