I completely agree: townlands belong in OSM. Historic boundaries that follow townland boundaries probably deserve more scrutiny. All the boundaries would be welcome in OHM, if we can figure out the logistics of mapping them there.
Yes, this mapping did begin a few years before OHM was passably viable. I don’t mean to cast blame on those who reached for the closest tool they had available at the time. That would be particularly hypocritical of me, as someone who pioneered the practice of hoarding historical data in old_name=*, amenity:historic=*, and demolished:building=*. However, more historic Irish census boundaries are still being mapped in OSM as recently as last year, probably because there’s already a critical mass of them in OSM. I’m making a stink now because we still have an opportunity to change course while folks are still actively involved in the project. I’m not personally going to delete anything, but I’m concerned that these features are on the chopping block the more visibility they get from renderers and QA tools.
Since you’ve mentioned this on a few occasions: I completely understand your frustration about OHM having lost your contributions in a hard drive failure eight years ago. If I had participated in the project by then, the experience probably would’ve demotivated me for years and soured me on the project forever. All I can offer is that, nowadays, we have more redundancy in cloud storage and are looking into additional partnerships for long-term backups of old planet files. Hopefully as time goes on, we can recover what was lost, even if it requires doing it the hard way.
The case I usually make is more like: someone is bound to redo all your work in OHM sooner or later. It might as well be you, rather than someone about to naïvely repeat all the mistakes you wish you hadn’t made! 
In seriousness, the benefit to mapping history in OHM is that you can say more about that history without having to constantly look over your shoulder and self-censor and apologize for it.
On a more practical level, iD supports academic/industry standards for encoding dates and times and can filter features by time period so you don’t get overwhelmed. The interactive time slider on the main map is a real crowd-pleaser, something you can’t do with OSM without knowing the ins and outs of QGIS.
Yes and no. One can certainly use OSM to discover these out-of-copyright maps and trace them all over again. And technically, since OHM is based in the U.S., one can probably make a convincing argument that purely factual data traced from these maps would also be in the public domain, so it can be imported outright. But I don’t push that line of argument too aggressively, because I don’t want people to get the idea that they can completely disregard the ODbL. OHM might not be bound by European database rights, but we want to be good partners with the OSM community.
Another problem is that many of these boundaries have been conflated with other features, including 3,562 kilometers of roadways, for which we don’t know the start and end dates. When you map a street in OHM without a start date, it shows up anachronistically when you move the time slider to Roman or prehistoric times. This is a great QA tool, but too many undated features get in the way when mapping in OHM, just as too many dated features get in the way when mapping in OSM.
You’ll notice I didn’t say who does the calling-out. When it comes to boundaries of any kind, past or present, it’s usually someone from outside the country who rushes to judgment. Ireland is the outlier in terms of its inclusion of historical boundaries in OSM. I just hope that Ireland won’t also be an outlier in OHM for a lack of boundaries because we’re trying too hard to shoehorn them into OSM.