Great idea for an independent line. You have a penchant for back stories (many modellers don’t actually take that seriously enough) and such a scenario would allow you to run appropriate stock. Consistency is key. Think what your board of directors’ policy was and why it was adopted. Eg SLNCR found that only 0-6-4T locos gave reliable running so all brand new locos were to that design. The tight curves etc of your line might be the reason that only UK industrials would be suitable for it. Come up with a suitable selection of rolling stock - 4w or 6w coaches, and light wagons, letter them G and SCR and you’re away. Focus on the scenery and buildings to make the layout look suitably Irish.