There can be a few reasons for this happening and it can be tricky to pin down why!
Space: Onion sets need space to develop into bulbs. Sets that are planted too close together may lack the space to develop bulbs - ideally sets should be 10cm apart. Similarly the bed should have a depth of at least 10cm. If growing in rows make sure they are 25-30cm apart. Another place things can go wrong if the bed is the right depth but the onion sets are planted too deep within it.
Nutrients: If the soil is very high in nitrogen/if a lot of plant food has been added, this can cause the leaves to develop at expense of the bulbs being formed.
Water: Onion sets need regular water to develop. Ireland can be deceptive in sometimes giving the impression there is a lot of rain but then following that up with periods that are not necessarily very warm but might be dry, reducing the water available to onions so they can develop.
Soil: Onion sets need a loose well draining fertile soil to develop. If the soil is compacted, rocky or heavy clay soil they may struggle to form bulbs.
Ph balance: It the Ph of the soil is off, onion bulbs won't form. Onions prefer slightly acidic soil. Not sure of the Ph? Use a tester
Weeds: Onion sets should be kept well weeded so they don't compete for nutrients - I doubt this is your particular issue though!
Pests: These can sometimes attack developing bulbs.
Temperature: While unlikely to stop bulbs forming, fluctuating temps may affect bulb size as the bulb starts to swell when consistently warmer temperatures arrive.
Time: Sets planted in October should be ready from July onwards to early autumn depending on when they were planted and local conditions. The sets shrink first to put energy into developing leaves, and then when the ground starts warming they swell again to develop bulbs. Be sure not to harvest too early - the challenge is to prevent bolting before the bulb is fully developed.