what are the main challenges Ireland faces as regards providing sustainable energy sources to meet future energy demand?
This is the biggest problem for most countries right now, and Ireland is no exception. We already have all the technology we need for 100% renewable electricity grids combined with the electrification of urban transport and heating. And for most countries, that's more economic than any alternative, once the externalities (pollution, liability cover) are priced in. What's missing is political will and vision. And that is, in part, driven by ...
Like most countries, Ireland has industrial incumbents with a lot of fixed assets (with sunk costs) that are dedicatd to highly-polluting ways to do things. So they carry a lot of economic weight, and consequentially political weight too.
One much larger neighbour
For most countries, large-capacity interconnectors to several neighbours provide the best option for balancing a grid with a high (>50%) proportion of exogenously-variable renewables such as wind, wave, tidal and solar: and Ireland's potential renewables are mostly of that kind, with relatively little scope for geothermal or storage hydro. However, Ireland, being on the continental periphery, has just one close neighbour, that is much larger than it. And that means the bigger the interconnectors, the more that prices on the all-Ireland grid would get driven largely by what's happening on the British grid, which makes it politically and economically challenging.
Balancing the grid at the scale of quarter-cycle to seconds
Exogenously-variable renewables such as wind don't necessarily bring much inertia to the grid, unlike thermal plant (and in particular coal). So the sort of balancing of supply and demand (aka balancing) that happens very easily on a grid with lots of fossil plant, at the scale of quarter-cycle (5ms on a 50Hz grid) to a few seconds, needs more planning and realigned incentives on a wind-rich grid. Technically, it's a solved problem - Quebec's grid, and others, have incentivised the innovation of within-turbine grid-balancing services at these time scales. What's missing, is a successful model for economic incentivisation to ensure the best balance of the maximisation of generated electricity, with sufficient balancing.
Rural, water and air transport
Rural, water and air transport remain challenges for most countries. Hydrocarbons provide a very high ratio of energy per unit mass, and per unit volume, and those are incredibly valuable for motorised transport. There are a bunch of options: biogenic hydrocarbons, synthetic hydrocarbons, hydrogen, some putative new chemical battery - but as yet there are no clear winners.
Things which had looked like challenges, but now have known solutions
Size of the available renewable resource
It used to be thought that that might be an issue, and some people got worried because energy per unit land area seemed to be low. That turned out to be a non-issue, once people did the maths. The island of Ireland's total energy consumption (electricity, heat, transport, etc) is about 21.5GW (15GW for Eire, 6.5GW for Northern Ireland). Its land area is 84 421 km2, and its sea area is well over 400 000 km2, meaning that even at the 2W/m2 that non-optimised onshore wind gives, sufficiency of the available resource is not a challenge: only a tiny fraction of Ireland's land and waters would need renewables, to meet all current energy consumption, and that's without any energy efficiency measures.
(I've no intention of causing conflict by considering the island as a whole: the grids of Eire and Northern Ireland are combined, and the same wind, sun, tides and waves reach both parts of the island. So it's just technically easier to analyse the island of Ireland as one whole entity)
Balancing the grid at the scale of minutes to days
Again, this used to be a problem, but now we're spoilt for solutions. Electricity interconnectors to neighbours do raise challenges of political economics, but in terms of capital investment they're very affordable, and provide a technical solution for most of the problem. The rest can be taken care of through sustainable biomass and power-to-gas. There has been a project to lobby for extensive pumped hydro storage in Eire, but the economics don't currently look favourable.
Forecastability of the weather
There are no countries immediately to the west or south-west of Ireland; and that's the direction that most of the weather comes in from. That means that Ireland has fewer opportunities than most of the rest of Europe to gather free weather information for 4-hour and 24-hour ahead forecasts. However, with the growing use of weather buoys, weather masts and floating LiDAR, complemented by satellite data, this is a solved problem.
As there currently aren't any sustainable biofuels that can cost competitively compete with other energy sources, I'm going to forgo discussing it and focus on carbon free energy sources.
My guess is that since Ireland has access to all carbon free forms of sustainable energy sources, there is nothing particularly different between the challenges Ireland would face vs. the rest of the world. That being said; all non-carbon based sustainable energy thus far, aside from geothermal, runs into the major problem of energy storage. Tidal, solar, and wind based energy generation is cyclic at best, and completely unpredictable and unreliable at worst.
Most current solutions seem to point toward better developing electro-fuels, hydrogen generation, and fly-wheel technology. But past solutions have involved such things as flooding mounting mine tunnels and then draining them though turbines to recapture some energy when it's at peak demand.
Decades of wind power subsidies haven't done much for energy solutions. The biggest lie of is that the wind can power an entire economy. Every country that’s ever tried has failed – abysmally. The UK is no exception nor Ireland. The UK in 2017 had Renewable Energy installations:Wind Power nameplate capacity of 19.0 Gigawatts: Actual capacity 19.6%: producing 3.72 GW. Because of both these poor capacity factors and their unreliability, the business case for Weather Dependent Renewables is not viable without the massive subsidies or fossil fuel backup for 80% of the time. Those additional costs are charged to power consumers.