Hi Seamus! The shields depicted in iconography are indeed smaller than the Viking ones, round with a central boss.
Here's some examples:
Book of Kells:


Shrine of the Stowe Missal

White Island sculpture

Southampton Psalter

High Cross of Kells


High Cross of Durrow

High Cross of Monasterboice


sculpture from Carndonagh, Co. Donegal

and here's a photo from the Dublin museum of a shield boss and a grip, description as follows: " Iron shield boss. Lagore, Co. Meath. W.4" "Copper alloy shield grip. River Bann, Co. Antrim/Derry 1939:63 "

plus there was a comment on the Irish shields underneath: "The copper alloy shield grip and iron boss although not found together, illustrate the small size of the Irish shield. These were in turn adopted by Viking settlers in the West and such small shields occur in Viking graves in Scotland and the Isle of Man".
The Irish shield bosses are mentioned in 'Weapons and Warfare in Viking and Medieval Dublin' by Andrew Halpin, in an Appendix 1 'Shield boss from Fishamble Street' by Stephen H.Harrison. While discussing its unusual for a Viking boss, conical rather than hemispherical shape, he notices
'With its pronounced apex point, the Fishamble Street may also have some more local parallels, specifically a group of Irish shield bosses from crannog sites, notably Lagore, Co. Meath and Lough Faughan, Co. Down, most of which have similar apices. Although none of these come from securely dated contexts, all are probably earlier than the present example. In addition, all known early medieval Irish shield bosses are very much smaller than the Fishamble Street boss, with diameters ranging from 7.7cm to 9.0cm, excluding the flange or rim. Small size is also characteristic of a group of conical shield bosses from the ninth-century 'Viking' burial complex at Kilmainham/Islandbridge, Dublin, which seem to have been strongly influenced by the small Irish shield bosses."
He refers to works:
Hencken, H.O'N. 1950 Lagore Crannog: an Irish royal residence of the seventh to tenth century AD.
Proceedings of the Royal Irish Academy 53C, 1-248
Collins, A.E.P. 1955 Excavations at Lough Faughan crannog, Co. Down 1951-2.
Ulster Journal of Archaeology 18, 45-82
Harrison, S.H. 1995 Viking Age shield bosses in Dublin and Irish Sea area. Unpublished MA thesis, National University of Ireland, University College Cork