My general rule is that you can string your harp with four different gauges of wire. All you need to know is what are the top and bottom notes of your harp, and how the low bass strings are tuned (with or without short-octave gaps). If you don’t know that, get in touch and we can work it out!

The strings above treble g′ are 0.48mm
yellow brass
or 0.48mm
soft iron
The octave above
na comhluighe /
sisters g is 0.6mm yellow brass
The octave below
na comhluighe /
sisters g is 0.75mm yellow brass
The lowest bass strings, below bass G are
yellow brass

I have detailed stringcharts available for certain standard models of harps. These are free to download from my website, or I can send you a printed copy for the cost of postage.
Hollybrook harp stringchart (2018)
Downhill harp string chart (2018)
NMI Carolan harp stringchart (2019)
Kildare harp stringchart (2020)
Clonalis Carolan harp stringchart (2020)
Castle Otway harp stringchart (2020)

If you want to read more about my thinking, and the rationale for stringing and setting up an old Irish harp in this way, you can read about this in two of my scholarly papers:
Simon Chadwick, ‘Stringing’, as part of the HHSI Hollybrook harp survey, HHSI 2021 (PDF download)
Simon Chadwick, ‘Provenance and recording of an eighteenth-century harp’, The Galpin Society Journal 73, March 2020, pages 103–5.

I have also worked out speculative stringcharts for medieval and Renaissance Gaelic harps; see my article ‘Medieval Gaelic harp setup’, in Early Music Performer, issue 40, Spring 2017 for my thinking on this. The following string charts are available as free PDF downloads:
Trinity College harp & Queen Mary harp stringchart (2016, revised 2019). Two harps on one chart, showing my suggested “medieval” setup with the harp tuned to c.
Lamont harp stringchart May 2018. Current state and reconstructed original state, showing my suggested “Renaissance” setup with the harp tuned to b♭.

If your harp is not one of the standard models, or you don’t know what the top and bottom note should be, I can do a string length analysis and make a custom stringchart for your harp, based on the measured string lengths. The cost of this is £20.