Robin,
Did you get that hieroglyph resolved yet?!
It is difficult to help you further without the explicit shape.
[Your two geometric descriptions seem to be mutually contradictory. Can you describe it more accurately and consistently? Else scan it, or sketch it (with e.g. MS Paint) ?]
Why do you refer to it as an Irish emblem?
[Such conjures up images of Celtic knots, Ogham scripts, Harps & Shamrocks. None of these seem to match your description particularly well.]
Think that you need to eliminate the (more probable) mundane first.
Are you sure that it is not simply an initial (of a middle name)?
Could it be an " O' " prefix to the surname?
[Such was much in vogue during "Gaelic Revival" times.]
If the record is part of a register list, then the symbol might simply be a place marker for an associated "Note" further down the page or later in the document.
In pre-computer days one way of marking such notes was graphically, using bold "X"s with associated dots.
[Such served the purpose of the super-numerals used for footnotes today.]
They were designed to boldly distinct (from the text) and clearly distinguishable from each other.
[To make cross-reference easy and unambiguous.]
I saw a good example of this in "Who Do You Think You Are" on TV this week. The context was in connection with an immigrants register for Brisbane, Oz in the late 19thC.
The husband's name was noted, but had a complex symbol nearby.
It was an "X" with an extra horizontal bar through it. Each of its six sectors had a dot "." placed within it.
At the foot of the page the symbol was repeated again. The applicable note was written next to it.
"No allocation of land to be made."
[From which could be deduced that he had previously immigrated. i.e. this was him returning with a second wife!]
I also saw similar uses of such symbols on "Wainwrights Fell Walks Guides" tonight, such books dating from the 1950s and 1960s. They were produced by him manually as scriptive text, with pen and ink, in a lifetime's labour of love.
He used "X" and "XX" to distinguish his (afterthought) notes.
Each had a single dot placed within the side and lower sectors, but not in the top ones.
[I have a vague recollection of having drawn such symbols myself, as a wee lad, last century ... not sure from where I learnt it!]
Hope this is of further help.
John