There are a few different bits that might create a more militrised Ireland, though there will always be limits to that. Lets face it ROI against the UK is only going to end one way.
From 1920 onwards, first I don't think the Free State could force/hold Ulster if there was resistance in Ulster against it, the State and it's forces were just too young for something like that in my view.
From 22 on, there are options, there was a What If program a while back on RTE that Belfast sent me the link to, one of the episodes was about Collins. In it they talked about him looking at the continent for options for the new State rather than just the UK. One of the things they mentioned was him suggesting a delegation sent to look at the Swiss model of service, and if it could be used in Ireland. Have him survive or just have the delegation already sent and supported in the Cabinet (maybe as something for Collin's legacy) and that gets you a form of Conscription. Follow through into WW2 and perhaps a more international Ireland looks at the neutral nations getting rolled over and decides that's not going to happen to Ireland and so starts fully investing in it's defences.
In OTL we bought 200+ Bren Carriers and a couple of LV60's perhaps the tank order is expanded to create a permanent armoured battalion strength (post War buy enough Comets and spares to support this, following on maybe look at whatever is cheap and cheerful to support it). Perhaps Ireland's own main defence exporter Timoney gets more State support and orders (ie, modern purchases of either the Bushmaster, or the Taiwanese CM-32 both ha significant design work done in Ireland (the CM-32 prototypes were done here)
If the Navy avoids being shut down the first time perhaps it could have a couple of WW1 hulls still in service by WW2, maybe see some anti sub action to prove how important they were. If not post WW2 it avoids being shut down again and gets the full order of 6 Flowers (perhaps as well get a couple of Minesweepers as we had the RN in Cork for 2 years post war doing this), this means twice as many ships, crew, support facilities, which means that going forward the capabilities could be more (if in OTL the Navy has managed to go from 3-0-8 hulls then from 6-10/12 might be doable if originally stronger). Better yard management and you could have at least 3 Ethine's with on board helicopter operations. IN OTL by the Crash we were looking at a modified Absalon class for UN operations perhaps greater UN op's might drive a cheap Expeditionary investment prior to that.
For air operations, perhaps have the UK/NATO firmly state that they had enough duties without covering Irish airspace combined with some Russian intrusions might encourage more investment in post Vampire/Fourga units. Perhaps more effort is made to get a more full up trainer system like either the Hawk, or depending on the relationship with the UK and time something like the Alpha. Ireland is never going to be able to support much larger than armed trainers, but these would be more than we have. Better investment in helicopters (perhaps more investment for Air/Sea rescue leading to bigger variants)
But leaving aside large scale changes, like I said there are small bits that could change things across the century. Not sure it gets you what you want but it improves Ireland a good bit, a less censored coverage of WW2 might draw attention to Ireland's needs, and perhaps has the Defence Forces treated better post war (not shut down the Navy again for 1
)
A better coverage/reporting/respect of the Irish forces and their actions in the Congo (the Siege of Jadotville), or coverage of the failure of the equipment they were deployed with, leading to more funding for the forces. Following on Ireland is in a better position to perform UN operations as we actually have the equipment perhaps?
A better ship building plan for the Navy (according to one of the managers there had been some interest in the Irish designs by the US Coastguard given our sea conditions, not sure if that's accurate), leading to a better operational capability and realisation of export potential, (ie if the Cork yard had better internal IR, they might have been able to complete the original Eithne's and been able to market a OPV/Research vessel design to nations).
If we hadn't had twits in Government during the boom, perhaps the Air Corps might have ended up with a dozen or so of Blackhawks instead of the AW 139's we have. (apparently the Blackhawk was considered "too military" for the Air Corps by the politicians
). Or moved earlier on more P50 buys/start the P60 builds before the combination of the crash and the yard having other work delayed them until 2014.
Edit: and Apparently we do have a foreign intelligence service, don't know about anything more than that but it's in the budget (or knowing us it could be a slush fund but given the inability of the Government to keep things like that secret it's more likely an actual thing.
