Valeman Billy, whose research was the catalyst for an ecumenical service on the Rock to mark the Feast of St Andrew of Scotland last year, says a clue may lie in the heraldry of the Earldom of Lennox.
And he will be one of those gathering again on the Rock for a similar all-ticket service, this time in honour of St Patrick, at 12 noon on Saturday, March 16, the day before the saint's feast day.
Billy said: "It is surely more than coincidence that the flag of Ireland, as incorporated in the Union Jack, bears the central feature of the heraldry of the Scottish earldom in which the patron saint of Ireland is traditionally believed to have been born."
He pointed out that the flag of St Patrick includes the central aspect of the Lennox Arms - a red saltire, a diagonal or St Andrew's-type cross, on a white background, as adopted by the Earls of Lennox around the 12th or 13th century.
Billy, a retired local government officer, added: "That red cross is the one incorporated into the Union Flag. It is the flag that is displayed on the ceiling above the central lobby of the House of Commons to represent Ireland as the flag of St Patrick along side the flags of St George of England, St David of Wales and St Andrew of Scotland.
"But it will take a great deal more research to establish how this red saltire came to be part of the Lennox coat of arms which was used by the old Dunbartonshire County Council. Although the Office of the Lord Lyon has referred to a belief that the red saltire was adopted by an early Earl of Lennox because he had undertaken a crusade, they admitted that the real reason for the Lennox choice of that emblem remains a matter of speculation.
"It is a coat of arms not on general display in this area now but it is still in some public buildings, including the main chapel at Cardross Crematorium, the Council Offices at Garshake, Dumbarton Police HQ and Hill Street Police Office, Alexandria."
The flag and coat of arms of the current local authority for the area, West Dunbartonshire Council, includes the red saltire and an image of St Patrick.
History was made when that first Christian service of worship since 1571 took place in November last year and church and civic representatives gathered on the site of an ancient chapel dedicated to St Patrick to celebrate the life of St Andrew, Scotland's patron saint.
Billy, who is the author of a historical novel called Upon This Rock, said the Rock is the oldest recorded fortified site in Britain and that "this has perhaps overshadowed the fact that it has also, since very early times, been a focus of Christian worship".
He added: "It is probable that Christianity came first to the Dumbarton area through soldiers or merchants of the Roman Empire.
"There are a number of historians who believe that St Patrick was the son of a paymaster attached to the Roman Legions who are also connected with this area through the Antonine Wall, which is now an official World Heritage Site and runs into Old Kilpatrick.
"It could well be that St Patrick, who around 450 AD sent a letter condemning slavery to Coroticus, who is believed to have been a king ruling from Alt Clut (an ancient name for Dumbarton Rock), was born here and taken into captivity here and sold into slavery in Ireland.
"A chapel dedicated to St Patrick on Dumbarton Rock is said to have been founded by St Modwenna in the 6th century, then the earliest reference to Dumbarton as the birthplace of St Patrick was in an 11th century manuscript.
"By the 12th century considerable numbers of pilgrims were visiting the saint's shrine at Kilpatrick, just a few miles from the Rock. By 1542 an English military document was referring to Dumbarton as the birthplace of St Patrick as a simple statement of fact.
"The reason for the choice of the red saltire as an Irish national flag is also a matter of debate. The strongest theory is that it was taken from the Order of St Patrick which was founded in 1783, and which adopted as its emblem a red saltire on white. But why was this symbol chosen by the Order of St Patrick? We would love to find out.
"Logic suggests that, either the red saltire on white was an ancient symbol of St Patrick, which was adopted by the Earls of Lennox and subsequently by the Order of St Patrick, or the heraldry of the Lennox - the district of St Patrick's birth - was adopted as a symbol of the saint - and then of Ireland.
"Or is all this simply coincidence? Many of us would consider the first option the most credible."
* Billy Scobie's book Upon This Rock by Alexander Tait is available for £7.99 from Books from Scotland.com