The Residential Tenancies Act does not cover who is responsible for lightbulbs.
However, if the landlord supplies light bulbs and the tenant takes them without replacing them, or damages them intentionally or carelessly, the tenant would be responsible for their replacement.
If the light bulbs remain in place but have blown, responsibility becomes less certain. Standard light bulbs that have blown may be seen as consumables and be replaceable by the tenant, or can be seen as fair wear and tear and be replaceable by the landlord.
Non-standard light bulbs that may be more expensive, or require special treatment to fit, may be the responsibility of the landlord. If a light bulb of this kind blows, it is likely to be treated as fair wear and tear.