Firstly, I am very sorry to hear that you have gone down with TB - it is heart-breaking.
Secondly - apologies while I proceed to write a Gospel. (I have worked in the UK for the last few years but have a fair idea of what is going on in Ireland too)
A few things can mitigate a TB breakdown
1. Residual infection (infection in the herd which hasn't been fully cleared out) - you have said this is unlikely since your herd hasnt had TB for years. Have these animals been in contact with any bought-in stock / neighbouring stock in their lifetime? ...remember that TB is a typically slow moving infection. (same family of bacteria as johne's disease in which a calf can be infected and not show up as positive until over 2 years old)
2. Neighbouring herds - you have said that tillage is all around, so unlikely cattle to cattle contact.
3. Slurry / Manure - have you used any slurry / manure from TB infected herds on your ground? (probably unlikely but TB can survive in faeces if conditions are favourable and faeces has not had time to decompose / not been stored for long enough prior to spreading)
4. Wildlife - certainly an option. consider the installation of a wildlife camera at several locations around the pasture (you can get them on Amazon for about £40 for a cheaper starter one)
Walk the field boundaries - you will be surprised that you can see tracks where badgers travel. Badgers go through areas of activity and dormancy and so setts can be active and dormant from year to year.
Badgers are attracted to high energy starchy stuff - so maize is certainly an attraction to them (especially when there is thick cover for them), they also like molasses containing concentrates?
Do you feed mineral licks on the ground? Do you feed meal on the ground or in a trough that could be accessed by badgers? Do you have low water troughs / stagnant water? (TB can survive in water for 60-90days if conditions are favourable)
Certainly deer is an option - however, they are not seen as the main intermediate host in the life cycle of bovine TB - but that said the exact risk from deer is unclear as there is little data on levels of TB excretion, local deer abundance and contact with cattle.
I would urge you to have a look at the following website for loads of information about TB (really sound, practical information and not jargon heavy)
https://www.tbknowledgeexchange.co.uk/tb-fact-sheets/
https://www.tbknowledgeexchange.co....KE_BovineTB_in_deer_and_other_UK_wildlife.pdf