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Summary Full report
McIlroy's Port to Portmuck - MineralogyAntrim
Site number:538
ESCC:EC
Locality Type:Coastal section Status: ASSI
Grid Reference: D454027 Centroid
Google maps: 54.85137,-5.73613
County: AntrimCouncil area:Mid & East Antrim Borough Council
Period:Tertiary
Lithostrat:Antrim Lava Group
Site Description
image: 0538a.jpg McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997.
McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997.
Highlights:
Sodalite-bearing pegmatite in picritic lava flow and unusual minerals in meta-basalt.
Introduction:
Geological features of McIlroy's Port have been described in detail by Preston (see Key Site 640 - McIlroy’s Port, Island Magee [Igneous/Tertiary Block]).
image: 0538b.jpg McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997.
McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997.
A layered lava flow with picritic base and a volcanic vent of basaltic fragments are main features of the site.
Description:
A sodalite-bearing pegmatite occurs near the base of the picritic lava flow, principally to the south of the vent. Preston notes that this is the first recorded occurrence of sodalite in Ireland.
image: 0538c.jpg McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997: pegmatite veins.
McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997: pegmatite veins.
The pegmatite is best developed as a 20-30cm thick layer just above high tide level from about 20m to 30m south of the stack which marks the southern margin of the vent. Closer to the vent, pegmatite forms 2-3cm veins through the lower 50cm of the picritic flow. Preston states that this pegmatite comprises c.70% titanaugite and 10-20% sodalite with minor amounts of plagioclase, magnetite and apatite. He also mentions that recent electron micro-probe studies have identified melilite, nepheline, perovskite and spinel.

image: 0538d.jpg McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997: course of dolerite dyke.
McIlroy’s Port to Portmuck, Islandmagee, Co.Antrim, June 1997: course of dolerite dyke.
Between this site and Portmuck, a line of stacks accessible at low tide marks the course of a dolerite dyke through the amygdaloidal basalts. Emeleus and Preston (1969) quote Walker (1948) as recording the occurrence of andradite, apatite, magnetite, soda diopside, ilmenite, sphene and calcite besides zeolites in contact metamorphosed amygdaloidal basalt next to the dyke. However, in fieldwork for the present survey, only natrolite was noted in these meta-basalts. Just seaward of the point where the dyke emerges onto the shoreline, a pipe-like dolerite intrusion some 4m in diameter has metamorphosed the adjoining chalk into marble.

McIlroy's Port is a rather inaccessible coastline locality c.1km north of Portmuck harbour. It can be approached from Portmuck at low tide, or by descending from farm lanes to the east. Neither approach is easy, and the preferred access route would be through re-instating a clifftop path from Portmuck, from which paths could be created descending into McIlroy's Port and into the bay at the agglomerate vent (see also Key Site 539 - Coast opposite Isle of Muck, Island Magee). An attractive waterfall is located some 50m north of the vent and access between these points is via a natural tunnel near high tide level. The boulder beach at the vent locality is hazardous when wet, as the boulders are very slippery.
Notes:
See also Key Site 640 - McIlroy's Port, Island Magee and Key Site 1215 - Portmuck, Island Magee.
Keywords
Minerals:Natrolite, Sodalite, Titanaugite
Rocks:Amydaloidal Basalt, Pegmatite, Picrite
Structures:dyke, volcanic vent
Access
Approach:McIlroy's Port can be approached from Portmuck at low tide or by descending from farm lanes to the east. Neither approach is easy, and the preferred access route would be through re-instating a clifftop path from Portmuck, from which paths could be created descending into McIlroy's Port and into the bay at the agglomerate vent.
Management:
Coastal cliffs accessible only at low tides. Improve access via clifftop path.
Record type: ESCR report Recorded by: R. Nawaz and N.R. Moles
Entered by: E M Porter
Last updated:07 MAY 68
 Enlander, I., Dempster, M. & Doughty, P., 2026. McIlroy's Port to Portmuck - Mineralogy, County Antrim, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=538. Accessed on 2026-11-06.

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