Clonmacnoise Bridge by: Aidan O’Sullivan
and Donal Boland

Fig.12.4.1: Location map of Clonmacnoise Bridge,
Co. Offaly [OSI]
The early medieval wooden bridge at
Clonmacnoise (Fig. 12.4.1) crosses the River
Shannon immediately downstream of the monastic
site (Plate 12.4.1) and is visible underwater
today as a double row of eroded wooden posts at
water depths of 1.5-5m. The site was first
discovered in 1994 by two local divers (Donal
Boland and Mattie Grehan). In 1995, a
preliminary site survey was carried out by the
Irish Underwater Archaeological Research Team
(IUART). In 1997 and 1998, a detailed site
survey and excavation was carried out by Aidan
O’Sullivan and Donal Boland (Management for
Archaeology Underwater Ltd.) for the National
Monuments Service (formally Dúchas) and
dendrochronological dates were obtained from the
School of Palaeoecology and Archaeology, Queen’s
University Belfast, indicating that the
structure was built c. 804 AD.

Plate
12.4.1: Aerial view looking south-west,
Clonmacnoise Bridge, Co. Offaly [National
Monuments Service]

Plate
12.4.2: Underwater photo showing timbers,
Clonmacnoise Bridge, Co. Offaly [Donal Boland]
The early medieval wooden bridge was located
on a shallow and relatively narrow part of the
river, but owing to the exceptional depth and
softness of the riverbed clays there, it could
never have served as a natural fording place.
While it is likely that this was a long-term
crossing point for boats, it was crossed by a
bridge structure for only a brief period. While
its proximity to the monastic site explains its
location, it is also significant that the bridge
was approached on both sides by more
long-distance route ways, the Slí Mhór
to the east and a bog trackway in Coolumber to
the west.
The bridge measured c.120m in length and 4-5m
in width. A line of narrow, vertical hazel and
alder poles was first driven into the riverbed
to mark out its proposed location. The main
bridge structure was then constructed of a
double row of vertical oak posts, 4-5m apart.
The bridge consisted of c. 25 pairs of
these posts, spaced at c. 4-6m
intervals going across the river, of which 17
survived. Underwater excavations revealed that
the vertical posts were prevented from sinking
into the riverbed by a simple, but ingenious
method of individual base-plates, beams and
planks (Plate 12.4.2) This could be
reconstructed from the single vertical post
excavated and extracted from the riverbed. This
was driven to a depth of 3.5m into the soft
clays. It had been crudely sharpened to a blunt
point with augur holes drilled through the tip.
It had a through-mortise cut through its side
(3.5m from the tip). This snugly held a
transverse oak beam; itself mortised at either
end. Each of these mortises also a held broad,
cleft oak plank. As the post descended into the
riverbed, this arrangement of planks prevented
it from sinking too far and allowed it to stand
without swaying. Similar features on most of the
other vertical posts indicate that this
technique was used right across the river. The
superstructure of the bridge has long been
destroyed by collapse and riverine erosion and
many timbers may have been robbed out soon after
its abandonment. It is unlikely however, that it
stood much higher than 6m above the riverbed,
with a walkway of hurdles, poles or planks
supported on a superstructure of jointed beams.
Comparative studies of early medieval bridges in
northern Europe and ethnological parallels of
more recent wooden bridges suggest that the
Clonmacnoise bridge would not have had a life
span of much more than 40-50 years.
The underwater surveys and excavations also
uncovered a number of associated finds. This
includes eleven dugout boats, some of which were
found beside the bridge timbers. Remarkably,
three of these boats contained early medieval
woodworking tools, including felling and
carpentry axes and a whetstone. These tools may
well have been lost during the bridge
construction project. Another find on the
riverbed beside the bridge included a rare
eighth/ninth century copper-alloy basin, similar
to examples found previously in the early
medieval Derrynaflan hoard and in Derreen, Co.
Clare. This is a type of vessel that was
probably used in the church liturgy, for holding
wine or washing of the hands. Its location on
the riverbed could be explained by its
accidental loss or by its violent removal from
the monastery during a raid. Alternatively, the
vessel may have been deliberately thrown into
the river (it bears damage from a sharp object)
for some as yet poorly understood reason. Other
finds include a prehistoric chert core, an iron
dish, wooden objects, animal bone and quantities
of iron slag eroded into the river from probable
early medieval ironworking sites along the
riverbank.
The early medieval bridge at Clonmacnoise
should be interpreted in the context of local
and regional social and political developments
in the Irish midlands in the early ninth century
AD. It may have been constructed to enable the
growing monastic population to travel easily
back and forth to their agricultural estates on
the west side of the river, or to enable
pilgrims to come easily to the monastery.
Alternatively, the bridge may have been
constructed through royal patronage, as part of
the aggressive political and military expansion
of Connacht kings during the period. It can also
be understood as a single event in the long-term
history of travel across the River Shannon.
Note: The Clonmacnoise bridge project will be
published in 2004 as O’Sullivan, A. and Boland,
D. The early medieval bridge at Clonmacnoise:
Underwater archaeological investigations,
1994-1998. Clonmacnoise Studies, Vol.
3, National Monuments Service.
References
Boland, D. 1996 ‘Clonmacnoise’,
In I. Bennett (ed.), Excavations 1995.
Wordwell Ltd, Bray.
Boland, D. and O’Sullivan, A. 1997 ‘An
early medieval wooden bridge at Clonmacnoise’,
In F.J.G. Mitchell and C. Delaney (eds.),
The Quaternary of the Irish Midlands. Field
Guide 21, Irish Association for Quaternary
Studies, Dublin, 14-21.
O’Sullivan, A. and Boland, D. 1998 ‘Clonmacnoise’,
In I. Bennett (ed.), Excavations 1997.
Wordwell, Bray, 148-49.
O’Sullivan, A. and Boland, D. 1998
‘Medieval Irish engineers bridge the River
Shannon’, Discovering Archaeology
1, 32-37.
O’Sullivan, A. and Boland, D. 2000 The
Clonmacnoise bridge: An early medieval river
crossing in County Offaly. Archaeology
Ireland Heritage Guide No. 11. Wordwell, Bray.
O’Sullivan, A., Brady, N. and Boland, D. 2000
‘Clonmacnoise’, In I. Bennett (ed.),
Excavations 1998. Wordwell, Bray, 174.
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